485BPOS

AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON NOVEMBER 15, 2012

FILE NO.  333-160918

FILE NO.  811-22321

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER THE SECURITIES ACT OF 1933  [X]

Post-Effective Amendment No.  35

AND

REGISTRATION STATEMENT

UNDER THE INVESTMENT COMPANY ACT OF 1940  [X]

Amendment No.  37

MAINSTAY FUNDS TRUST

(exact name of registrant as specified in charter)

51 MADISON AVENUE, NEW YORK, NEW YORK 10010

(address of principal executive office)

REGISTRANT'S TELEPHONE NUMBER: (212) 576-7000

J. Kevin Gao, Esq. MainStay Funds Trust 169 Lackawanna Avenue Parsippany, NJ 07054 Copy to: Sander M. Bieber, Esq. Dechert LLP 1775 I Street, NW Washington, DC 20006

(NAME AND ADDRESS OF AGENT FOR SERVICE)

It is proposed that this filing will become effective

x Immediately upon filing pursuant to paragraph (b) of Rule 485

o on ________ pursuant to paragraph (b)(1) of Rule 485

o 60 days after filing pursuant to paragraph (a)(1) of Rule 485

o on ________, pursuant to paragraph (a)(1) of Rule 485

o 75 days after filing pursuant to paragraph (a)(2) of Rule 485

o on  ________  pursuant to paragraph (a)(2) 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

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 under Rule 485(b) under the Securities Act and that it has duly caused this Post-Effective Amendment No. 35 to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Parsippany in the State of New Jersey, on the 15^th day of November, 2012.

MAINSTAY FUNDS TRUST
By:	/s/ Stephen P. Fisher
	Stephen P. Fisher
	President and Principal Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 35 to the Registration Statement has been signed below by the following persons in the capacities indicated on November 15, 2012.

SIGNATURE	TITLE
/s/ Stephen P. Fisher	President and Principal Executive Officer
Stephen P. Fisher
/s/ Susan B. Kerley*	Trustee and Chairman of the Board
Susan B. Kerley
/s/ John Y. Kim*	Trustee
John Y. Kim
/s/ Alan R. Latshaw*	Trustee
Alan R. Latshaw
/s/ Peter Meenan*	Trustee
Peter Meenan
/s/ Richard H. Nolan, Jr.*	Trustee
Richard H. Nolan, Jr.
/s/ Richard S. Trutanic*	Trustee
Richard S. Trutanic
/s/ Roman L. Weil*	Trustee
Roman L. Weil
/s/ John A. Weisser*	Trustee
John A. Weisser
/s/ Jack R. Benintende	Treasurer and Principal Financial
Jack R. Benintende	and Accounting Officer
*By: /s/ J. Kevin Gao
J. Kevin Gao
As Attorney-in-Fact

*    PURSUANT TO POWERS OF ATTORNEY PREVIOUSLY FILED

EXHIBIT INDEX

EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
ProspectusDate	rr_ProspectusDate	Nov. 02, 2012
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	MainStay Cornerstone Growth Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Fund seeks long-term growth of capital.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the MainStay Funds. This amount may vary depending on the MainStay Fund in which you invest. More information about these and other discounts is available from your financial professional and in the "Information on Sales Charges" section starting on page 16 of the Prospectus and in the "Alternative Sales Arrangements" section on page 63 of the Statement of Additional Information.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
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 may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These 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 114% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	114.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the MainStay Funds. This amount may vary depending on the MainStay Fund in which you invest.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	50,000
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The 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 then redeem all of your shares at the end of those periods (except as indicated with respect to Class C shares). The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same.
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund invests primarily in equity securities of U.S. companies, the prospective earnings growth of which, in the opinion of Cornerstone Capital Management, LLC, the Fund's Subadvisor, is not fully appreciated by the market or reflected in current market valuations. The Subadvisor also looks for companies it believes have strong management, superior industry positions, excellent balance sheets and superior earnings growth  potential. U.S. companies are companies organized in the U.S. that trade primarily on U.S. securities markets. The Fund may also invest in foreign companies. Generally, foreign companies are companies organized outside the U.S. and that trade primarily in non-U.S. securities markets. Under normal circumstances, the Fund will invest at least 80% of its assets in common stocks of large-capitalization growth companies. Large-capitalization growth companies are those that, at the time of investment, have market capitalizations within the range of market capitalizations of companies appearing in the Russell 1000® Growth Index (which ranged from $220.32 million to $623.60 billion as of August 31, 2012). The Fund generally will invest in common stocks of companies with market capitalizations of at least $3 billion at the time of purchase. Investment Process. Normally, the Fund holds between 35 and 55 securities. The 25 most highly regarded of these companies, in the Subadvisor's opinion, usually constitute approximately 70% or more of the Fund's net assets. Notwithstanding this focus, the Fund has no policy to concentrate in securities of issuers in a particular industry or group of industries. Although the Fund does not have a policy to concentrate its investments in any one industry, a large portion of its assets has historically been in technology companies which the Subadvisor believes offer strong growth potential. During market declines, while adding to positions in favored stocks, the Fund becomes somewhat more aggressive, gradually reducing the number of companies represented in its portfolio. Conversely, in rising markets, while reducing or eliminating fully valued positions, the Fund becomes somewhat more conservative, gradually increasing the number of companies represented in its portfolio. Through this process, the Subadvisor tends to add to positions on price weakness and sell into price strength, all else being equal and assuming long-term company fundamentals are intact. Risk is therefore increased during periods of weakness and reduced during periods of strength. The Subadvisor uses this active management strategy to attempt to add incremental performance while seeking to mitigate risk by utilizing a buy low, sell high discipline.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Loss of Money Risk: Before considering an investment in the Fund, you should understand that you could lose money. Market Changes Risk: The value of the Fund's investments may change because of broad changes in the markets in which the Fund invests, which could cause the Fund to underperform other funds with similar objectives. From time to time, markets may experience periods of acute stress that may result in increased volatility. Such market conditions tend to add significantly to the risk of short-term volatility in the net asset value of the Fund's shares. Additionally, during periods of market stress, a company's access to adequate financing may be impaired, requiring it to raise capital on terms which could dilute the value of the Fund's investment. Management Risk: The investment strategies, practices and risk analysis used by the Fund's Subadvisor may not produce the desired results. Equity Securities Risk: Investments in common stocks and other equity securities are particularly subject to the risk of changing economic, stock market, industry and company conditions and the risks inherent in the portfolio manager's ability to anticipate such changes that can adversely affect the value of the Fund's holdings. Opportunity for greater gain often comes with greater risk of loss. Growth Stock Risk: If growth companies do not increase their earnings at a rate expected by investors, the market price of the stock may decline significantly, even if earnings show an absolute increase. Growth company stocks also typically lack the dividend yield that can cushion stock prices in market downturns. Foreign Securities Risk: Investments in foreign securities may be riskier than investments in U.S. securities. Differences between U.S. and foreign regulatory regimes and securities markets, including less stringent investor protections and disclosure standards of some foreign markets, less liquid trading markets, as well as political and economic developments in foreign countries, may affect the value of the Fund's investments in foreign securities. Foreign securities may also subject the Fund's investments to changes in currency rates. Concentrated Portfolio Risk: Because the Fund invests in relatively few holdings, a larger percentage of its assets may be invested in a particular issuer or in fewer companies than is typical of other mutual funds. This may increase volatility. The Fund will be more susceptible to adverse economic, political, regulatory or market developments affecting a single issuer. Technology Stock Risk: The Fund may focus its investments in technology companies. Technology companies are subject to risks such as those relating to the potential rapid obsolescence of technology, failure of the market to accept new technologies and difficulty obtaining financing for necessary research and development or expansion.
Risk Lose Money [Text]	rr_RiskLoseMoney	Before considering an investment in the Fund, you should understand that you could lose money.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Past Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and tables indicate some of the risks of investing in the Fund. The bar chart shows you how the Fund's calendar year performance has varied over the life of the Fund. Sales loads are not reflected in the bar chart. If they were, returns would be less than those shown. The average annual total returns table shows how the Fund's average annual total returns (before and after taxes) for the one-year period, five-year period and life of the Fund compare to those of two broad-based securities market indices. The Fund has selected the Russell 1000® Growth Indexas its primary benchmark. The Russell 1000® Growth Index measures the performance of the large-cap growth segment of the U.S. equity universe. It includes those Russell 1000® companies with higher price-to-book ratios and higher forecasted growth values. The Fund has selected the S&P 500® Index as its secondary benchmark. The S&P 500® Index is widely regarded as the standard index for measuring large-cap U.S. stock market performance. Performance figures for Class A and Class I shares reflect the historical performance of the Class A and Class I shares of the Keystone Large Cap Growth Fund (the predecessor to the Fund, which was subject to a different fee structure), for periods prior to January 11, 2013. Performance data for the classes varies based on differences in their fee and expense structures and the length of time these share classes have been offered. The Fund commenced operations, and Class A shares were first offered, on August 7, 2006. Class I shares were first offered on November 2, 2009. Investor Class shares, Class B shares, Class C shares and Class R2 shares were first offered on January 14, 2013 and include the historical performance of Class A shares through January 11, 2013. Performance information for Investor Class shares, Class B shares, Class C shares and Class R2 shares have been adjusted for differences in fees and expenses. Unadjusted, the performance for the newer classes might have been different. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Please visit mainstayinvestments.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and tables indicate some of the risks of investing in the Fund.
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	mainstayinvestments.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Annual Returns, Class A Shares (by calendar year 2007-2011)
Bar Chart Narrative [Text Block]	rr_BarChartNarrativeTextBlock	The bar chart shows you how the Fund's calendar year performance has varied over the life of the Fund. Sales loads are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales loads are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter 3Q/10    18.72% Worst Quarter 4Q/08   -20.96% As of September 15, 2012, the Class A shares of the Fund had a year-to-date return 20.80%.
Index No Deduction for Fees, Expenses, Taxes [Text]	rr_IndexNoDeductionForFeesExpensesTaxes	reflects no deductions 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	Actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns are not relevant if you hold your shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns shown are for Class A shares. After-tax returns for the other share classes may vary.
Performance Table Explanation after Tax Higher	rr_PerformanceTableExplanationAfterTaxHigher	In some cases, the return after taxes may exceed the return before taxes due to an assumed tax benefit from any losses on a sale of shares at the end of the measurement period.
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. In some cases, the return after taxes may exceed the return before taxes due to an assumed tax benefit from any losses on a sale of shares at the end of the measurement period. Actual after-tax returns depend on your tax situation and may differ from those shown. After-tax returns are not relevant if you hold your shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns shown are for Class A shares. After-tax returns for the other share classes may vary.
Caption	rr_AverageAnnualReturnCaption	Average Annual Total Returns (for the periods ended December 31, 2011)
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Russell 1000® Growth Index
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Russell 1000® Growth Index (reflects no deductions for fees, expenses, or taxes)
1 Year	rr_AverageAnnualReturnYear01	2.64%
5 Years	rr_AverageAnnualReturnYear05	2.50%
Since Inception	rr_AverageAnnualReturnSinceInception	4.57%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | S&P 500®Index
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	S&P 500® Index (reflects no deductions for fees, expenses, or taxes)
1 Year	rr_AverageAnnualReturnYear01	2.11%
5 Years	rr_AverageAnnualReturnYear05	(0.25%)
Since Inception	rr_AverageAnnualReturnSinceInception	1.90%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Investor Class
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.50%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.70%	[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.63%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.58%	[3]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	702
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	1,021
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,363
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	2,325
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Investor Class
1 Year	rr_AverageAnnualReturnYear01	(5.97%)
5 Years	rr_AverageAnnualReturnYear05	0.67%
Since Inception	rr_AverageAnnualReturnSinceInception	2.91%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class A
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.50%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.70%	[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.37%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.32%	[3]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	677
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	945
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,234
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	2,053
Annual Return 2007	rr_AnnualReturn2007	16.10%
Annual Return 2008	rr_AnnualReturn2008	(43.21%)
Annual Return 2009	rr_AnnualReturn2009	43.80%
Annual Return 2010	rr_AnnualReturn2010	15.94%
Annual Return 2011	rr_AnnualReturn2011	(0.50%)
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 15, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	20.80%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2010
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	18.72%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(20.96%)
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class A
1 Year	rr_AverageAnnualReturnYear01	(5.97%)
5 Years	rr_AverageAnnualReturnYear05	0.67%
Since Inception	rr_AverageAnnualReturnSinceInception	2.91%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class A | After Taxes on Distributions
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions Class A
1 Year	rr_AverageAnnualReturnYear01	(6.70%)
5 Years	rr_AverageAnnualReturnYear05	(0.03%)
Since Inception	rr_AverageAnnualReturnSinceInception	2.21%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class A | After Taxes on Distributions and Sales
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions and Sale of Fund Shares Class A
1 Year	rr_AverageAnnualReturnYear01	(2.99%)
5 Years	rr_AverageAnnualReturnYear05	0.28%
Since Inception	rr_AverageAnnualReturnSinceInception	2.19%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class B
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	5.00%
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.70%	[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other Expenses	rr_OtherExpensesOverAssets	0.63%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	2.33%	[3]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	736
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	1,027
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,445
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	2,479
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	236
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	727
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	1,245
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	2,479
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class B
1 Year	rr_AverageAnnualReturnYear01	(6.18%)
5 Years	rr_AverageAnnualReturnYear05	0.66%
Since Inception	rr_AverageAnnualReturnSinceInception	3.05%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class C
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.70%	[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other Expenses	rr_OtherExpensesOverAssets	0.63%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	2.33%	[3]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	336
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	727
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	1,245
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	2,666
Expense Example, No Redemption, 1 Year	rr_ExpenseExampleNoRedemptionYear01	236
Expense Example, No Redemption, 3 Years	rr_ExpenseExampleNoRedemptionYear03	727
Expense Example, No Redemption, 5 Years	rr_ExpenseExampleNoRedemptionYear05	1,245
Expense Example, No Redemption, 10 Years	rr_ExpenseExampleNoRedemptionYear10	2,666
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class C
1 Year	rr_AverageAnnualReturnYear01	(2.23%)
5 Years	rr_AverageAnnualReturnYear05	1.05%
Since Inception	rr_AverageAnnualReturnSinceInception	3.21%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class I
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.70%	[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.37%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.07%	[3]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	109
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	340
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	590
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,306
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class I
1 Year	rr_AverageAnnualReturnYear01	(0.25%)
5 Years	rr_AverageAnnualReturnYear05
Since Inception	rr_AverageAnnualReturnSinceInception	11.00%
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class R2
Risk Return [Abstract]	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fees (as an annual percentage of the Fund's average daily net assets)	rr_ManagementFeesOverAssets	0.70%	[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.47%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.42%	[3]
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	145
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	449
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	776
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	1,702
Label	rr_AverageAnnualReturnLabel	Return Before Taxes Class R2
1 Year	rr_AverageAnnualReturnYear01	(0.60%)
5 Years	rr_AverageAnnualReturnYear05	1.71%
Since Inception	rr_AverageAnnualReturnSinceInception	3.89%

[1] A contingent deferred sales charge of 1.00% may be imposed on certain redemptions made within one year of the date of purchase on shares that were purchased without an initial sales charge.

[2] The management fee is as follows: 0.70% on assets up to $500 million; 0.675% on assets from $500 million to $1 billion; and 0.65% on assets over $1 billion.

[3] Effective upon completion of the reorganization of the Keystone Large Cap Growth Fund, a series of Keystone Mutual Funds, with and into the Fund, New York Life Investments has contractually agreed to waive fees and/or reimburse expenses so that Class A and Class I shares of the Fund's Total Annual Fund Operating Expenses (excluding taxes, interest, litigation, extraordinary expenses, brokerage and other transaction expenses relating to the purchase or sale of portfolio investments, and acquired (underlying) fund fees and expenses) do not exceed the following percentages of average daily net assets: Class A, 1.34%; and Class I, 1.09%. This agreement will be in effect for a two-year period, unless extended by New York Life Investments and approved by the Board of Trustees.

XML

MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund

MainStay Cornerstone Growth Fund

Investment Objective

The Fund seeks long-term growth of capital.

Fees and Expenses of the Fund

The table below describes the fees and expenses that you may pay if you buy and
hold shares of the Fund. You may qualify for sales charge discounts if you and
your family invest, or agree to invest in the future, at least $50,000 in the
MainStay Funds. This amount may vary depending on the MainStay Fund in which you
invest. More information about these and other discounts is available from your
financial professional and in the "Information on Sales Charges" section starting
on page 16 of the Prospectus and in the "Alternative Sales Arrangements" section
on page 63 of the Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees MainStay Cornerstone Growth Fund	Investor Class		Class A		Class B	Class C	Class I	Class R2
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	5.50%		5.50%		none	none	none	none
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	none	[1]	none	[1]	5.00%	1.00%	none	none

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

Annual Fund Operating Expenses MainStay Cornerstone Growth Fund		Investor Class	Class A	Class B	Class C	Class I	Class R2
Management Fees (as an annual percentage of the Fund's average daily net assets)	[1]	0.70%	0.70%	0.70%	0.70%	0.70%	0.70%
Distribution and/or Service (12b-1) Fees		0.25%	0.25%	1.00%	1.00%	none	0.25%
Other Expenses		0.63%	0.37%	0.63%	0.63%	0.37%	0.47%
Total Annual Fund Operating Expenses	[2]	1.58%	1.32%	2.33%	2.33%	1.07%	1.42%

Example

The 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 then redeem all
of your shares at the end of those periods (except as indicated with respect to
Class C shares). The Example also assumes that your investment has a 5% return
each year and that the Fund's operating expenses remain the same.

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example MainStay Cornerstone Growth Fund (USD $)	Investor Class	Class A	Class B	Class C	Class I	Class R2
Expense Example, with Redemption, 1 Year	702	677	736	336	109	145
Expense Example, with Redemption, 3 Years	1,021	945	1,027	727	340	449
Expense Example, with Redemption, 5 Years	1,363	1,234	1,445	1,245	590	776
Expense Example, with Redemption, 10 Years	2,325	2,053	2,479	2,666	1,306	1,702

Expense Example, No Redemption MainStay Cornerstone Growth Fund (USD $)	Class B	Class C
Expense Example, No Redemption, 1 Year	236	236
Expense Example, No Redemption, 3 Years	727	727
Expense Example, No Redemption, 5 Years	1,245	1,245
Expense Example, No Redemption, 10 Years	2,479	2,666

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 may
indicate higher transaction costs and may result in higher taxes when Fund
shares are held in a taxable account. These 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 114%
of the average value of its portfolio.

Principal Investment Strategies

The Fund invests primarily in equity securities of U.S. companies, the prospective
earnings growth of which, in the opinion of Cornerstone Capital Management, LLC,
the Fund's Subadvisor, is not fully appreciated by the market or reflected in
current market valuations. The Subadvisor also looks for companies it believes
have strong management, superior industry positions, excellent balance sheets and
superior earnings growth  potential. U.S. companies are companies organized in
the U.S. that trade primarily on U.S. securities markets. The Fund may also invest
in foreign companies. Generally, foreign companies are companies organized outside
the U.S. and that trade primarily in non-U.S. securities markets.

Under normal circumstances, the Fund will invest at least 80% of its assets in
common stocks of large-capitalization growth companies. Large-capitalization
growth companies are those that, at the time of investment, have market
capitalizations within the range of market capitalizations of companies
appearing in the Russell 1000® Growth Index (which ranged from $220.32 million
to $623.60 billion as of August 31, 2012). The Fund generally will invest in
common stocks of companies with market capitalizations of at least $3 billion at
the time of purchase.

Investment Process. Normally, the Fund holds between 35 and 55 securities. The
25 most highly regarded of these companies, in the Subadvisor's opinion, usually
constitute approximately 70% or more of the Fund's net assets. Notwithstanding
this focus, the Fund has no policy to concentrate in securities of issuers in a
particular industry or group of industries. Although the Fund does not have a
policy to concentrate its investments in any one industry, a large portion of
its assets has historically been in technology companies which the Subadvisor
believes offer strong growth potential.

During market declines, while adding to positions in favored stocks, the Fund
becomes somewhat more aggressive, gradually reducing the number of companies
represented in its portfolio. Conversely, in rising markets, while reducing or
eliminating fully valued positions, the Fund becomes somewhat more conservative,
gradually increasing the number of companies represented in its portfolio.
Through this process, the Subadvisor tends to add to positions on price weakness
and sell into price strength, all else being equal and assuming long-term
company fundamentals are intact. Risk is therefore increased during periods of
weakness and reduced during periods of strength. The Subadvisor uses this active
management strategy to attempt to add incremental performance while seeking to
mitigate risk by utilizing a buy low, sell high discipline.

Principal Risks

Loss of Money Risk: Before considering an investment in the Fund, you should
understand that you could lose money.

Market Changes Risk: The value of the Fund's investments may change because of
broad changes in the markets in which the Fund invests, which could cause the
Fund to underperform other funds with similar objectives. From time to time,
markets may experience periods of acute stress that may result in increased
volatility. Such market conditions tend to add significantly to the risk of
short-term volatility in the net asset value of the Fund's shares. Additionally,
during periods of market stress, a company's access to adequate financing may be
impaired, requiring it to raise capital on terms which could dilute the value of
the Fund's investment.

Management Risk: The investment strategies, practices and risk analysis used by
the Fund's Subadvisor may not produce the desired results.

Equity Securities Risk: Investments in common stocks and other equity securities
are particularly subject to the risk of changing economic, stock market, industry
and company conditions and the risks inherent in the portfolio manager's ability
to anticipate such changes that can adversely affect the value of the Fund's
holdings. Opportunity for greater gain often comes with greater risk of loss.

Growth Stock Risk: If growth companies do not increase their earnings at a rate
expected by investors, the market price of the stock may decline significantly,
even if earnings show an absolute increase. Growth company stocks also typically
lack the dividend yield that can cushion stock prices in market downturns.

Foreign Securities Risk: Investments in foreign securities may be riskier than
investments in U.S. securities. Differences between U.S. and foreign regulatory
regimes and securities markets, including less stringent investor protections
and disclosure standards of some foreign markets, less liquid trading markets,
as well as political and economic developments in foreign countries, may affect
the value of the Fund's investments in foreign securities. Foreign securities
may also subject the Fund's investments to changes in currency rates.

Concentrated Portfolio Risk: Because the Fund invests in relatively few holdings,
a larger percentage of its assets may be invested in a particular issuer or in
fewer companies than is typical of other mutual funds. This may increase
volatility. The Fund will be more susceptible to adverse economic, political,
regulatory or market developments affecting a single issuer.

Technology Stock Risk: The Fund may focus its investments in technology companies.
Technology companies are subject to risks such as those relating to the potential
rapid obsolescence of technology, failure of the market to accept new technologies
and difficulty obtaining financing for necessary research and development or
expansion.

Past Performance

The following bar chart and tables indicate some of the risks of investing in
the Fund. The bar chart shows you how the Fund's calendar year performance has
varied over the life of the Fund. Sales loads are not reflected in the bar chart.
If they were, returns would be less than those shown. The average annual total
returns table shows how the Fund's average annual total returns (before and
after taxes) for the one-year period, five-year period and life of the Fund
compare to those of two broad-based securities market indices. The Fund has
selected the Russell 1000® Growth Indexas its primary benchmark. The Russell
1000® Growth Index measures the performance of the large-cap growth segment of
the U.S. equity universe. It includes those Russell 1000® companies with higher
price-to-book ratios and higher forecasted growth values. The Fund has selected
the S&P 500® Index as its secondary benchmark. The S&P 500® Index is widely
regarded as the standard index for measuring large-cap U.S. stock market
performance.

Performance figures for Class A and Class I shares reflect the historical
performance of the Class A and Class I shares of the Keystone Large Cap Growth
Fund (the predecessor to the Fund, which was subject to a different fee structure),
for periods prior to January 11, 2013. Performance data for the classes varies
based on differences in their fee and expense structures and the length of time
these share classes have been offered. The Fund commenced operations, and Class
A shares were first offered, on August 7, 2006. Class I shares were first offered
on November 2, 2009. Investor Class shares, Class B shares, Class C shares and
Class R2 shares were first offered on January 14, 2013 and include the historical
performance of Class A shares through January 11, 2013.

Performance information for Investor Class shares, Class B shares, Class C shares
and Class R2 shares have been adjusted for differences in fees and expenses.
Unadjusted, the performance for the newer classes might have been different. Past
performance (before and after taxes) is not necessarily an indication of how the
Fund will perform in the future. Please visit mainstayinvestments.com for more
recent performance information.

The bar chart shows you how the Fund's calendar year performance has
varied over the life of the Fund. Sales loads are not reflected in
the bar chart. If they were, returns would be less than those shown.

Annual Returns, Class A Shares (by calendar year 2007-2011)

Best Quarter
3Q/10    18.72%
Worst Quarter
4Q/08   -20.96%

As of September 15, 2012, the Class A shares of the Fund had a year-to-date
return 20.80%.

Average Annual Total Returns (for the periods ended December 31, 2011)

Average Annual Total Returns MainStay Cornerstone Growth Fund	Label	1 Year	5 Years	Since Inception
Investor Class	Return Before Taxes Investor Class	(5.97%)	0.67%	2.91%
Class A	Return Before Taxes Class A	(5.97%)	0.67%	2.91%
Class A After Taxes on Distributions	Return After Taxes on Distributions Class A	(6.70%)	(0.03%)	2.21%
Class A After Taxes on Distributions and Sales	Return After Taxes on Distributions and Sale of Fund Shares Class A	(2.99%)	0.28%	2.19%
Class B	Return Before Taxes Class B	(6.18%)	0.66%	3.05%
Class C	Return Before Taxes Class C	(2.23%)	1.05%	3.21%
Class I	Return Before Taxes Class I	(0.25%)		11.00%
Class R2	Return Before Taxes Class R2	(0.60%)	1.71%	3.89%
Russell 1000® Growth Index	Russell 1000® Growth Index (reflects no deductions for fees, expenses, or taxes)	2.64%	2.50%	4.57%
S&P 500®Index	S&P 500® Index (reflects no deductions for fees, expenses, or taxes)	2.11%	(0.25%)	1.90%

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.
In some cases, the return after taxes may exceed the return before taxes due to
an assumed tax benefit from any losses on a sale of shares at the end of the
measurement period. Actual after-tax returns depend on your tax situation and may
differ from those shown. After-tax returns are not relevant if you hold your
shares through tax-deferred arrangements, such as 401(k) plans or individual
retirement accounts. After-tax returns shown are for Class A shares. After-tax
returns for the other share classes may vary.

[1] A contingent deferred sales charge of 1.00% may be imposed on certain redemptions made within one year of the date of purchase on shares that were purchased without an initial sales charge.

[1] The management fee is as follows: 0.70% on assets up to $500 million; 0.675% on assets from $500 million to $1 billion; and 0.65% on assets over $1 billion.

[2] Effective upon completion of the reorganization of the Keystone Large Cap Growth Fund, a series of Keystone Mutual Funds, with and into the Fund, New York Life Investments has contractually agreed to waive fees and/or reimburse expenses so that Class A and Class I shares of the Fund's Total Annual Fund Operating Expenses (excluding taxes, interest, litigation, extraordinary expenses, brokerage and other transaction expenses relating to the purchase or sale of portfolio investments, and acquired (underlying) fund fees and expenses) do not exceed the following percentages of average daily net assets: Class A, 1.34%; and Class I, 1.09%. This agreement will be in effect for a two-year period, unless extended by New York Life Investments and approved by the Board of Trustees.

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Jun. 30, 2012
Registrant Name	dei_EntityRegistrantName	MAINSTAY FUNDS TRUST
Central Index Key	dei_EntityCentralIndexKey	0001469192
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Nov. 05, 2012
Document Effective Date	dei_DocumentEffectiveDate	Nov. 05, 2012
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Investor Class
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	KLGNX
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class A
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	KLGAX
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class B
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	KLGBX
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class C
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	KLGCX
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class I
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	KLGIX
MainStay Cornerstone Growth Fund (Prospectus Summary) | MainStay Cornerstone Growth Fund | Class R2
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	KLGRX