485BPOS

As filed with the Securities and Exchange Commission on November 18, 2013

1933 Act File No. 33-37615

1940 Act File No. 811-6174

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

POST-EFFECTIVE AMENDMENT NO. 37

AND

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940

AMENDMENT NO. 41

MFSÒ INSTITUTIONAL TRUST

(Exact Name of Registrant as Specified in Charter)

111 Huntington Avenue, Boston, Massachusetts 02199-7618

(Address of Principal Executive Offices)

Registrant’s Telephone Number, Including Area Code: (617) 954-5000

Susan S. Newton, Massachusetts Financial Services Company

111 Huntington Avenue, Boston, Massachusetts 02199-7618

(Name and Address of Agent for Service)

APPROXIMATE DATE OF PROPOSED PUBLIC OFFERING:

It is proposed that this filing will become effective (check appropriate box)

x immediately upon filing pursuant to paragraph (b)

o on [date] pursuant to paragraph (b)

o 60 days after filing pursuant to paragraph (a)(i)

o on [date] pursuant to paragraph (a)(i)

o 75 days after filing pursuant to paragraph (a)(ii)

o on [date] pursuant to paragraph (a)(ii) of rule 485.

If appropriate, check the following box:

o this post-effective amendment designates a new effective date for a previously filed post-effective amendment

NOTICE

Notice is hereby given that this Registration Statement has been executed on behalf of the Registrant by an officer of the Registrant as an officer and not individually, and the obligations of or arising out of this Registration Statement are not binding upon any of the Trustees, officers, or shareholders of the Registrant individually, but are binding only upon the assets and property of the Registrant.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment to the Registration Statement to be signed on its behalf by the undersigned, thereto duly authorized, in the City of Boston and The Commonwealth of Massachusetts on the 18th day of November 2013.

MFS® INSTITUTIONAL TRUST
By:	JOHN M. CORCORAN*
Name:	John M. Corcoran
Title:	President

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment to its Registration Statement has been signed below by the following persons in the capacities indicated on November 18, 2013.

SIGNATURE	TITLE
JOHN M. CORCORAN*	President (Principal Executive Officer)
John M. Corcoran
DAVID L. DILORENZO*	Principal Financial and Accounting Officer
David L. DiLorenzo
ROBERT E. BUTLER*	Trustee
Robert E. Butler
MAUREEN R. GOLDFARB*	Trustee
Maureen R. Goldfarb
DAVID H. GUNNING*	Trustee
David H. Gunning
WILLIAM R. GUTOW*	Trustee
William R. Gutow

MICHAEL HEGARTY*	Trustee
Michael Hegarty
JOHN P. KAVANAUGH*	Trustee
John P. Kavanaugh
ROBERT J. MANNING*	Trustee
Robert J. Manning
J. DALE SHERRATT*	Trustee
J. Dale Sherratt
LAURIE J. THOMSEN*	Trustee
Laurie J. Thomsen
ROBERT W. UEK*	Trustee
Robert W. Uek
	*By:	SUSAN S. NEWTON
	Name:	Susan S. Newton
		as Attorney-in-fact
	Executed by Susan S. Newton on behalf of those indicated pursuant to a Power of Attorney, dated December 7, 2012 (Trustees), and a Power of Attorney, dated December 7, 2012 (Corcoran) (DiLorenzo); filed herewith.

MFS Series Trust I

MFS Series Trust II

MFS Series Trust III

MFS Series Trust IV

MFS Series Trust V

MFS Series Trust VI

MFS Series Trust VII

MFS Series Trust VIII

MFS Series Trust IX

MFS Series Trust X

MFS Series Trust XI

MFS Series Trust XII

MFS Series Trust XIII

MFS Series Trust XIV

MFS Series Trust XV

MFS Series Trust XVI

Massachusetts Investors Growth Stock Fund

Massachusetts Investors Trust

MFS Institutional Trust

MFS Municipal Series Trust

MFS Variable Insurance Trust

MFS Variable Insurance Trust II

MFS Variable Insurance Trust III

MFS California Municipal Fund

MFS Charter Income Trust

MFS Government Markets Income Trust

MFS High Income Municipal Trust

MFS High Yield Municipal Trust

MFS InterMarket Income Trust I

MFS Intermediate High Income Fund

MFS Intermediate Income Trust

MFS Investment Grade Municipal Trust

MFS Multimarket Income Trust

MFS Municipal Income Trust

MFS Special Value Trust

(each a “Registrant”)

POWER OF ATTORNEY

The undersigned, a Trustee of each of the above-mentioned Registrants, hereby severally constitutes and appoints Mark N. Polebaum, Susan S. Newton,

Christopher R. Bohane, Thomas H. Connors, Brian E. Langenfeld and Susan A. Pereira, and each of them singly, as true and lawful attorneys, with full power to them and each of them to sign for each of the undersigned, in the names of, and in the capacities indicated below, any Registration Statement and any and all amendments thereto and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission for the purpose of registering the Registrant as a management investment company under the Investment Company Act of 1940 and/or the shares issued by the Registrant under the Securities Act of 1933 granting unto my said attorneys, and each of them, acting alone, full power and authority to do and perform each and every act and thing requisite or necessary or desirable to be done in the premises, as fully to all intents and purposes as he or she might or could do in person, hereby ratifying and confirming all that said attorneys or any of them may lawfully do or cause to be done by virtue thereof.

IN WITNESS WHEREOF, the undersigned have hereunto set their hand on this 7th day of December, 2012.

ROBERT E. BUTLER	Trustee
Robert E. Butler
MAUREEN R. GOLDFARB	Trustee
Maureen R. Goldfarb
DAVID H. GUNNING	Trustee
David H. Gunning
WILLIAM R. GUTOW	Trustee
William R. Gutow
MICHAEL HEGARTY	Trustee
Michael Hegarty
JOHN P. KAVANAUGH	Trustee
John P. Kavanaugh

ROBERT J. MANNING	Trustee
Robert J. Manning
J. DALE SHERRATT	Trustee
J. Dale Sherratt
LAURIE J. THOMSEN	Trustee
Laurie J. Thomsen
ROBERT W. UEK	Trustee
Robert W. Uek

MFS Series Trust I

MFS Series Trust II

MFS Series Trust III

MFS Series Trust IV

MFS Series Trust V

MFS Series Trust VI

MFS Series Trust VII

MFS Series Trust VIII

MFS Series Trust IX

MFS Series Trust X

MFS Series Trust XI

MFS Series Trust XII

MFS Series Trust XIII

MFS Series Trust XIV

MFS Series Trust XV

MFS Series Trust XVI

Massachusetts Investors Growth Stock Fund

Massachusetts Investors Trust

MFS Institutional Trust

MFS Municipal Series Trust

MFS Variable Insurance Trust

MFS Variable Insurance Trust II

MFS Variable Insurance Trust III

MFS California Municipal Fund

MFS Charter Income Trust

MFS Government Markets Income Trust

MFS High Income Municipal Trust

MFS High Yield Municipal Trust

MFS InterMarket Income Trust I

MFS Intermediate High Income Fund

MFS Intermediate Income Trust

MFS Investment Grade Municipal Trust

MFS Multimarket Income Trust

MFS Municipal Income Trust

MFS Special Value Trust

(each a “Registrant”)

POWER OF ATTORNEY

The undersigned, being the President and Principal Executive Officer, and Treasurer and Principal Financial and Accounting Officer, respectively, of each of the above-mentioned Registrants, hereby severally constitutes and appoints Mark N. Polebaum, Susan S. Newton, Christopher R. Bohane, Thomas H. Connors, Brian E.

Langenfeld and Susan A. Pereira, and each of them singly, as true and lawful attorneys, with full power to them and each of them to sign for each of the undersigned, in the names of, and in the capacities indicated below, any Registration Statement and any and all amendments thereto and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission for the purpose of registering the Registrant as a management investment company under the Investment Company Act of 1940 and/or the shares issued by the Registrant under the Securities Act of 1933 granting unto my said attorneys, and each of them, acting alone, full power and authority to do and perform each and every act and thing requisite or necessary or desirable to be done in the premises, as fully to all intents and purposes as he or she might or could do in person, hereby ratifying and confirming all that said attorneys or any of them may lawfully do or cause to be done by virtue thereof.

IN WITNESS WHEREOF, the undersigned have hereunto set their hand on this 7th day of December, 2012.

JOHN M. CORCORAN	President (Principal Executive Officer)
John M. Corcoran
DAVID L. DILORENZO	Principal Financial and Accounting Officer
David L. DiLorenzo

MFS® Institutional Trust

MFSÒ INSTITUTIONAL INTERNATIONAL EQUITY FUND

MFSÒ INSTITUTIONAL LARGE CAP VALUE FUND

INDEX TO EXHIBITS

EXHIBIT NO.	DESCRIPTION OF EXHIBIT	PAGE NO.
101	Risk/Return summary of each Fund’s prospectus as an Interactive Data File using eXtensible Business Reporting Language (“XBRL”)

XML

MFS Institutional Large Cap Value Fund

MFS® Institutional Large Cap Value Fund-Summary of Key Information

Investment Objective

The fund’s investment objective is to seek capital appreciation.

Fees and Expenses

This table describes the fees and expenses that you may pay when you buy and hold shares of the fund.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):

Annual Fund Operating Expenses	MFS Institutional Large Cap Value Fund
Management Fee	0.55%
Distribution and/or Service (12b-1) Fees	none
Other Expenses	0.10%
Total Annual Fund Operating Expenses	0.65%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds.

The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods; your investment has a 5% return each year; and the fund’s operating expenses remain the same.

Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Expense Example (USD $)	1 YEAR	3 YEARS	5 YEARS	10 YEARS
MFS Institutional Large Cap Value Fund	66	208	362	810

Portfolio Turnover

The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance.  During the most recent fiscal year, the fund’s portfolio turnover rate was 19% of the average value of its portfolio.

Principal Investment Strategies

MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests at least 80% of the fund’s net assets in issuers with large market capitalizations. MFS generally defines large market capitalization issuers as issuers with market capitalizations of at least $5 billion at the time of purchase.

MFS normally invests the fund’s assets primarily in U.S. equity securities. Equity securities include common stocks, preferred stocks, securities convertible into stocks, equity interests in real estate investment trusts (REITs), and depositary receipts for such securities.

MFS focuses on investing the fund’s assets in the stocks of companies it believes are undervalued compared to their perceived worth (value companies).

MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers. Quantitative models that systematically evaluate issuers may also be considered.

Principal Risks

As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

The principal risks of investing in the fund are:

Stock Market/Company Risk:  Stock markets are volatile and can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, and other conditions, as well as to investor perceptions of these conditions. The price of an equity security can decrease significantly in response to these conditions, and these conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the market in general.

Value Company Risk:  The stocks of value companies can continue to be undervalued for long periods of time and not realize their expected value and can be more volatile than the market in general.

Investment Selection Risk:  MFS’ investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests.

Liquidity Risk:  It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.

Performance Information

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of a broad measure of market performance.

The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-877-960-6077.

Bar Chart.

The total return for the nine-month period ended September 30, 2013, was 22.94%. During the period(s) shown in the bar chart, the highest quarterly return was 16.86% (for the calendar quarter ended June 30, 2009) and the lowest quarterly return was (19.05)% (for the calendar quarter ended December 31, 2008).

Performance Table.

Average Annual Total Returns (for the Periods Ended December 31, 2012)

Average Annual Returns	Label	Average Annual Returns, 1 Year	Average Annual Returns, 5 Years	Average Annual Returns, 10 Years
MFS Institutional Large Cap Value Fund	Fund Shares Returns Before Taxes	16.42%	1.17%	8.05%
MFS Institutional Large Cap Value Fund After Taxes on Distributions	Fund Shares Returns Before Taxes	15.72%	0.56%	6.77%
MFS Institutional Large Cap Value Fund After Taxes on Distributions and Sale of Fund Shares	Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares	11.61%	0.77%	6.63%
MFS Institutional Large Cap Value Fund Russell 1000 Value Index	Index Comparison (Reflects no deduction for fees, expenses or taxes) Russell 1000 Value Index	17.51%	0.59%	7.38%

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Prospectus Date	rr_ProspectusDate	Oct. 28, 2013

XML

MFS Institutional International Equity Fund

MFS® Institutional International Equity Fund- Summary of Key Information

Investment Objective

The fund’s investment objective is to seek capital appreciation.

Fees and Expenses

This table describes the fees and expenses that you may pay when you buy and hold shares of the fund.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):

Annual Fund Operating Expenses	MFS Institutional International Equity Fund
Management Fee	0.69%
Distribution and/or Service (12b-1) Fees	none
Other Expenses	0.05%
Total Annual Fund Operating Expenses	0.74%

Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds.

The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods; your investment has a 5% return each year; and the fund’s operating expenses remain the same.

Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Expense Example (USD $)	1 YEAR	3 YEARS	5 YEARS	10 YEARS
MFS Institutional International Equity Fund	76	237	411	918

Portfolio Turnover

The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance.  During the most recent fiscal year, the fund’s portfolio turnover rate was 18% of the average value of its portfolio.

Principal Investment Strategies

MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests at least 80% of the fund’s net assets in equity securities.

Equity securities include common stocks, preferred stocks, securities convertible into stocks, equity interests in real estate investment trusts (REITs), and depositary receipts for such securities.

MFS normally invests the fund’s assets primarily in foreign securities, including emerging market securities.

MFS may invest a large percentage of the fund’s assets in issuers in a single country, a small number of countries, or a particular geographic region.

In selecting investments for the fund, MFS is not constrained to any particular investment style. MFS may invest the fund’s assets in the stocks of companies it believes to have above average earnings growth potential compared to other companies (growth companies), in the stocks of companies it believes are undervalued compared to their perceived worth (value companies), or in a combination of growth and value companies.

MFS may invest the fund’s assets in companies of any size.

MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers. Quantitative models that systematically evaluate issuers may also be considered.

Principal Risks

As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

The principal risks of investing in the fund are:

Stock Market/Company Risk:  Stock markets are volatile and can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, and other conditions, as well as to investor perceptions of these conditions. The price of an equity security can decrease significantly in response to these conditions, and these conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the market in general.

Foreign and Emerging Markets Risk:  Exposure to foreign markets, especially emerging markets, through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, and other conditions. These factors can make foreign investments, especially those in emerging markets, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Emerging markets can have less developed markets, greater custody and operational risk, less developed legal, regulatory, and accounting systems, and greater political, social, and economic instability than developed markets.

Currency Risk:  The value of foreign currencies relative to the U.S. dollar fluctuates in response to market, economic, industry, political, regulatory, geopolitical, and other conditions, and a decline in the value of a foreign currency versus the U.S. dollar reduces the value in U.S. dollars of investments denominated in that foreign currency.

Geographic Concentration Risk: The fund’s performance could be closely tied to the market, currency, economic, political, regulatory, geopolitical, and other conditions in the countries or regions in which the fund invests and could be more volatile than the performance of more geographically-diversified funds.

Investment Selection Risk:  MFS’ investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests.

Liquidity Risk:  It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.

Performance Information

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of a broad measure of market performance and one or more other measures of performance for markets in which the fund may invest.

The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-877-960-6077.

Bar Chart.

The total return for the nine-month period ended September 30, 2013, was 12.36%. During the period(s) shown in the bar chart, the highest quarterly return was 21.13% (for the calendar quarter ended September 30, 2009) and the lowest quarterly return was (20.30)% (for the calendar quarter ended September 30, 2011).

Performance Table.

Average Annual Total Returns (for the Periods Ended December 31, 2012)

Average Annual Returns	Label	Average Annual Returns, 1 Year	Average Annual Returns, 5 Years	Average Annual Returns, 10 Years
MFS Institutional International Equity Fund	Fund Shares Returns Before Taxes	22.55%	1.27%	10.09%
MFS Institutional International Equity Fund After Taxes on Distributions	Fund's Shares Returns After Taxes on Distributions	22.40%	1.06%	9.40%
MFS Institutional International Equity Fund After Taxes on Distributions and Sale of Fund Shares	Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares	15.07%	1.11%	8.82%
MFS Institutional International Equity Fund MSCI EAFE (Europe, Australasia, Far East) Index (gross div)	Index Comparison (Reflects no deduction for fees, expenses or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (gross div)	17.90%	(3.21%)	8.70%
MFS Institutional International Equity Fund MSCI EAFE (Europe, Australasia, Far East) Index (net div)	Index Comparison (Reflects no deduction for fees, expenses or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (net div)	17.32%	(3.69%)	8.21%

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

MFS® Institutional Large Cap Value Fund-Summary of Key Information

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund’s investment objective is to seek capital appreciation.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay when you buy and hold shares of the fund.

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance.  During the most recent fiscal year, the fund’s portfolio turnover rate was 19% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

19.00%

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds.

The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods; your investment has a 5% return each year; and the fund’s operating expenses remain the same.

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests at least 80% of the fund’s net assets in issuers with large market capitalizations. MFS generally defines large market capitalization issuers as issuers with market capitalizations of at least $5 billion at the time of purchase.

MFS normally invests the fund’s assets primarily in U.S. equity securities. Equity securities include common stocks, preferred stocks, securities convertible into stocks, equity interests in real estate investment trusts (REITs), and depositary receipts for such securities.

MFS focuses on investing the fund’s assets in the stocks of companies it believes are undervalued compared to their perceived worth (value companies).

MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers. Quantitative models that systematically evaluate issuers may also be considered.

Risk [Heading]

rr_RiskHeading

Principal Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

The principal risks of investing in the fund are:

Stock Market/Company Risk:  Stock markets are volatile and can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, and other conditions, as well as to investor perceptions of these conditions. The price of an equity security can decrease significantly in response to these conditions, and these conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the market in general.

Value Company Risk:  The stocks of value companies can continue to be undervalued for long periods of time and not realize their expected value and can be more volatile than the market in general.

Investment Selection Risk:  MFS’ investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests.

Liquidity Risk:  It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.

Risk Lose Money [Text]

rr_RiskLoseMoney

As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund.

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance Information

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of a broad measure of market performance.

The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-877-960-6077.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund's performance over time and how the fund's performance over time compares with that of a broad measure of market performance.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-877-960-6077

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

mfs.com

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The fund's past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Bar Chart.

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

The total return for the nine-month period ended September 30, 2013, was 22.94%. During the period(s) shown in the bar chart, the highest quarterly return was 16.86% (for the calendar quarter ended June 30, 2009) and the lowest quarterly return was (19.05)% (for the calendar quarter ended December 31, 2008).

Year to Date Return, Label

rr_YearToDateReturnLabel

The total return for the nine-month period ended

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2013

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

22.94%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

highest quarterly return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

16.86%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

lowest quarterly return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(19.05%)

Performance Table Heading

rr_PerformanceTableHeading

Performance Table.

Index No Deduction for Fees, Expenses, Taxes [Text]

rr_IndexNoDeductionForFeesExpensesTaxes

(Reflects no deduction for fees, expenses or taxes)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

Performance Table Closing [Text Block]

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After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

Caption

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Average Annual Total Returns (for the Periods Ended December 31, 2012)

Russell 1000 Value Index

Risk/Return:

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Label

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Index Comparison (Reflects no deduction for fees, expenses or taxes) Russell 1000 Value Index

Average Annual Returns, 1 Year

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17.51%

Average Annual Returns, 5 Years

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0.59%

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

7.38%

MFS Institutional Large Cap Value Fund

Risk/Return:

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Management Fee

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0.55%

Distribution and/or Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.10%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.65%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

$ 66

Expense Example, with Redemption, 3 Years

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208

Expense Example, with Redemption, 5 Years

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362

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

$ 810

Annual Return 2003

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25.63%

Annual Return 2004

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15.98%

Annual Return 2005

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6.84%

Annual Return 2006

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21.38%

Annual Return 2007

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8.28%

Annual Return 2008

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(32.34%)

Annual Return 2009

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20.99%

Annual Return 2010

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11.30%

Annual Return 2011

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(0.08%)

Annual Return 2012

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16.42%

Label

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Fund Shares Returns Before Taxes

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

16.42%

Average Annual Returns, 5 Years

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1.17%

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

8.05%

MFS Institutional Large Cap Value Fund | After Taxes on Distributions

Risk/Return:

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Label

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Fund Shares Returns Before Taxes

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

15.72%

Average Annual Returns, 5 Years

rr_AverageAnnualReturnYear05

0.56%

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

6.77%

MFS Institutional Large Cap Value Fund | After Taxes on Distributions and Sale of Fund Shares

Risk/Return:

rr_RiskReturnAbstract

Label

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Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

11.61%

Average Annual Returns, 5 Years

rr_AverageAnnualReturnYear05

0.77%

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

6.63%

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

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MFS® Institutional International Equity Fund- Summary of Key Information

Objective [Heading]

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Investment Objective

Objective, Primary [Text Block]

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The fund’s investment objective is to seek capital appreciation.

Expense [Heading]

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Fees and Expenses

Expense Narrative [Text Block]

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This table describes the fees and expenses that you may pay when you buy and hold shares of the fund.

Operating Expenses Caption [Text]

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Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):

Portfolio Turnover [Heading]

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Portfolio Turnover

Portfolio Turnover [Text Block]

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The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance.  During the most recent fiscal year, the fund’s portfolio turnover rate was 18% of the average value of its portfolio.

Portfolio Turnover, Rate

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18.00%

Expense Example [Heading]

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Example

Expense Example Narrative [Text Block]

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This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds.

The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods; your investment has a 5% return each year; and the fund’s operating expenses remain the same.

Expense Example by, Year, Caption [Text]

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Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Strategy [Heading]

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Principal Investment Strategies

Strategy Narrative [Text Block]

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MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests at least 80% of the fund’s net assets in equity securities.

Equity securities include common stocks, preferred stocks, securities convertible into stocks, equity interests in real estate investment trusts (REITs), and depositary receipts for such securities.

MFS normally invests the fund’s assets primarily in foreign securities, including emerging market securities.

MFS may invest a large percentage of the fund’s assets in issuers in a single country, a small number of countries, or a particular geographic region.

In selecting investments for the fund, MFS is not constrained to any particular investment style. MFS may invest the fund’s assets in the stocks of companies it believes to have above average earnings growth potential compared to other companies (growth companies), in the stocks of companies it believes are undervalued compared to their perceived worth (value companies), or in a combination of growth and value companies.

MFS may invest the fund’s assets in companies of any size.

MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers. Quantitative models that systematically evaluate issuers may also be considered.

Risk [Heading]

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Principal Risks

Risk Narrative [Text Block]

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As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

The principal risks of investing in the fund are:

Stock Market/Company Risk:  Stock markets are volatile and can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, and other conditions, as well as to investor perceptions of these conditions. The price of an equity security can decrease significantly in response to these conditions, and these conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the market in general.

Foreign and Emerging Markets Risk:  Exposure to foreign markets, especially emerging markets, through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, and other conditions. These factors can make foreign investments, especially those in emerging markets, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Emerging markets can have less developed markets, greater custody and operational risk, less developed legal, regulatory, and accounting systems, and greater political, social, and economic instability than developed markets.

Currency Risk:  The value of foreign currencies relative to the U.S. dollar fluctuates in response to market, economic, industry, political, regulatory, geopolitical, and other conditions, and a decline in the value of a foreign currency versus the U.S. dollar reduces the value in U.S. dollars of investments denominated in that foreign currency.

Geographic Concentration Risk: The fund’s performance could be closely tied to the market, currency, economic, political, regulatory, geopolitical, and other conditions in the countries or regions in which the fund invests and could be more volatile than the performance of more geographically-diversified funds.

Investment Selection Risk:  MFS’ investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests.

Liquidity Risk:  It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.

Risk Lose Money [Text]

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As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund.

Risk Not Insured Depository Institution [Text]

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An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

Bar Chart and Performance Table [Heading]

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Performance Information

Performance Narrative [Text Block]

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The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of a broad measure of market performance and one or more other measures of performance for markets in which the fund may invest.

The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-877-960-6077.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund's performance over time and how the fund's performance over time compares with that of a broad measure of market performance and one or more other measures of performance for markets in which the fund may invest.

Performance Availability Phone [Text]

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1-877-960-6077

Performance Availability Website Address [Text]

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mfs.com

Performance Past Does Not Indicate Future [Text]

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The fund's past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future.

Bar Chart [Heading]

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Bar Chart.

Bar Chart Closing [Text Block]

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The total return for the nine-month period ended September 30, 2013, was 12.36%. During the period(s) shown in the bar chart, the highest quarterly return was 21.13% (for the calendar quarter ended September 30, 2009) and the lowest quarterly return was (20.30)% (for the calendar quarter ended September 30, 2011).

Year to Date Return, Label

rr_YearToDateReturnLabel

The total return for the nine-month period ended

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2013

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

12.36%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

highest quarterly return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Sep. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

21.13%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

lowest quarterly return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2011

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(20.30%)

Performance Table Heading

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Performance Table.

Index No Deduction for Fees, Expenses, Taxes [Text]

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(Reflects no deduction for fees, expenses or taxes)

Performance Table Uses Highest Federal Rate

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After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns (for the Periods Ended December 31, 2012)

MSCI EAFE (Europe, Australasia, Far East) Index (gross div)

Risk/Return:

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Label

rr_AverageAnnualReturnLabel

Index Comparison (Reflects no deduction for fees, expenses or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (gross div)

Average Annual Returns, 1 Year

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17.90%

Average Annual Returns, 5 Years

rr_AverageAnnualReturnYear05

(3.21%)

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

8.70%

MSCI EAFE (Europe, Australasia, Far East) Index (net div)

Risk/Return:

rr_RiskReturnAbstract

Label

rr_AverageAnnualReturnLabel

Index Comparison (Reflects no deduction for fees, expenses or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (net div)

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

17.32%

Average Annual Returns, 5 Years

rr_AverageAnnualReturnYear05

(3.69%)

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

8.21%

MFS Institutional International Equity Fund

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.69%

Distribution and/or Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.05%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.74%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

$ 76

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

237

Expense Example, with Redemption, 5 Years

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411

Expense Example, with Redemption, 10 Years

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$ 918

Annual Return 2003

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31.02%

Annual Return 2004

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18.86%

Annual Return 2005

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12.92%

Annual Return 2006

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27.94%

Annual Return 2007

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9.11%

Annual Return 2008

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(34.95%)

Annual Return 2009

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33.12%

Annual Return 2010

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10.97%

Annual Return 2011

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(9.55%)

Annual Return 2012

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22.55%

Label

rr_AverageAnnualReturnLabel

Fund Shares Returns Before Taxes

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

22.55%

Average Annual Returns, 5 Years

rr_AverageAnnualReturnYear05

1.27%

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

10.09%

MFS Institutional International Equity Fund | After Taxes on Distributions

Risk/Return:

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Label

rr_AverageAnnualReturnLabel

Fund's Shares Returns After Taxes on Distributions

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

22.40%

Average Annual Returns, 5 Years

rr_AverageAnnualReturnYear05

1.06%

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

9.40%

MFS Institutional International Equity Fund | After Taxes on Distributions and Sale of Fund Shares

Risk/Return:

rr_RiskReturnAbstract

Label

rr_AverageAnnualReturnLabel

Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

15.07%

Average Annual Returns, 5 Years

rr_AverageAnnualReturnYear05

1.11%

Average Annual Returns, 10 Years

rr_AverageAnnualReturnYear10

8.82%

XML

Document and Entity Information	0 Months Ended
	Jun. 30, 2013
Risk/Return:
Document Type	485BPOS
Document Period End Date	Jun. 30, 2013
Registrant Name	MFS INSTITUTIONAL TRUST
Central Index Key	0000867969
Amendment Flag	false
Document Creation Date	Oct. 25, 2013
Document Effective Date	Oct. 28, 2013
Prospectus Date	Oct. 28, 2013
MFS Institutional International Equity Fund | MFS Institutional International Equity Fund
Risk/Return:
Trading Symbol	MIEIX
MFS Institutional Large Cap Value Fund | MFS Institutional Large Cap Value Fund
Risk/Return:
Trading Symbol	ILVAX