MainStay MacKay International Equity Fund (Prospectus Summary) | MainStay MacKay International Equity Fund

MainStay MacKay International Equity Fund (formerly known as MainStay International Equity 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 invest at least $250,000 in Class T shares of the Fund in a single transaction. More information about these and other discounts is available from your financial professional and in the "Information on Sales Charges" section starting on page 96 of the Prospectus and in the "Alternative Sales Arrangements" section on page 108 of the Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees	MainStay MacKay International Equity Fund Class T
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	2.50%
Maximum Deferred Sales Charge (Load) (as a percentage of the lesser of the original offering price or redemption proceeds)	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 MacKay International Equity Fund Class T
Management Fees (as an annual percentage of the Fund's average daily net assets)	0.89%	[1]
Distribution and/or Service (12b-1) Fees	0.25%
Other Expenses	0.20%
Total Annual Fund Operating Expenses	1.34%

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 whether or not you redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same. The Example reflects the contractual fee waiver and/or expense reimbursement arrangement, if applicable, for the current duration of the arrangement only. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
| MainStay MacKay International Equity Fund | Class T | USD ($)	383	664	966	1,822

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 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 45% of the average value of its portfolio.

Principal Investment Strategies

The Fund invests in those companies that meet the quality and valuation criteria of MacKay Shields LLC, the Fund's Subadvisor.

The Fund normally invests at least 80% of its assets (net assets plus any borrowings for investment purposes) in equity securities of issuers, wherever organized, which operate mainly outside the U.S. The Fund invests in securities of companies which conduct business in a variety of countries, with a minimum of five countries other than the U.S. This includes countries with established economies as well as emerging market countries that the Subadvisor believes present favorable opportunities. The Fund may also invest in exchange-traded funds ("ETFs") to obtain this exposure or for other investment purposes.

Investment Process: The Subadvisor seeks to identify investment opportunities through “bottom-up” analysis and fundamental research. The Subadvisor performs research to identify reasonably priced companies with competitive market advantages that it believes are able to benefit from long-term market trends and that the Subadvisor believes are able to sustainably grow earnings over time regardless of economic climate. Allocations to countries and industries are also a result of the "bottom-up" stock selection process and, as a result, may deviate from the country and industry weightings in the benchmark. The Fund may not perform as well as its peers or benchmark during periods when the stock market favors the securities of businesses with low-quality earnings.

Generally, the Fund seeks to limit its investments in securities of: (i) any one company; (ii) companies in the same industry; (iii) companies located in any one country; and (iv) companies located in emerging markets (currently limited to 25% of the Fund's assets measured at the time of investment).

The Subadvisor may sell a security if it no longer believes the security will contribute to meeting the investment objective of the Fund. In considering whether to sell a security, the Subadvisor may evaluate, among other things, whether the security has approached full valuation, if the investment thesis is invalidated, if superior opportunities to redeploy exist or emerge, or if industry group or country weights or individual positions need to be adjusted.

Principal Risks

You can lose money by investing 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 investments selected by the Fund's Subadvisor may underperform the market or other investments. The Fund may receive large purchase or redemption orders which may have adverse effects on performance if the Fund were required to sell securities, invest cash or hold a relatively large amount of cash at times when it would not otherwise do so.

The principal risks of investing in the Fund are summarized below.

Market Risk: The value of the Fund's investments may fluctuate because of changes in the markets in which the Fund invests, which could cause the Fund to underperform other funds with similar investment objectives and strategies. Changes in these markets may be rapid and unpredictable. From time to time, markets may experience periods of stress for potentially prolonged periods that may result in: (i) increased market volatility; (ii) reduced market liquidity; and (iii) increased redemptions of Fund shares. Such conditions may add significantly to the risk of volatility in the net asset value of the Fund's shares.

Portfolio Management Risk: The investment strategies, practices and risk analysis used by the Subadvisor may not produce the desired results. In addition, the Fund may not achieve its investment objective, including during a period in which the Subadvisor takes temporary positions in response to unusual or adverse market, economic or political conditions, or other unusual or abnormal circumstances.

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 managers' 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.

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 and 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. Changes in the value of foreign currencies may make the return on an investment go up or down, unrelated to the quality or performance of the investment itself.

Liquidity and Valuation Risk: Securities purchased by the Fund may be illiquid at the time of purchase or liquid at the time of purchase and subsequently become illiquid due to, among other things, events relating to the issuer of the securities, market events, operational issues, economic conditions, investor perceptions or lack of market participants. The lack of an active trading market may make it difficult to sell or obtain an accurate price for a security. If market conditions or issuer specific developments make it difficult to value securities, the Fund may value these securities using more subjective methods, such as fair value pricing. In such cases, the value determined for a security could be different than the value realized upon such security's sale. As a result, an investor could pay more than the market value when buying Fund shares or receive less than the market value when selling Fund shares. This could affect the proceeds of any redemption or the number of shares an investor receives upon purchase. Liquidity risk may also refer to the risk that the Fund may not be able to pay redemption proceeds within the allowable time period because of unusual market conditions, unusually high volume of redemptions, or other reasons. To meet redemption requests or to raise cash to pursue other investment opportunities, the Fund may be forced to sell securities at an unfavorable time and/or under unfavorable conditions, which may adversely affect the Fund.

Emerging Markets Risk: The risks related to investing in foreign securities are generally greater with respect to securities of companies that conduct their business activities in emerging markets or whose securities are traded principally in emerging markets. The risks of investing in emerging markets include the risks of illiquidity, increased price volatility, smaller market capitalizations, less government regulation, less extensive and less frequent accounting, financial and other reporting requirements, risk of loss resulting from problems in share registration and custody, substantial economic and political disruptions and the nationalization of foreign deposits or assets.

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.

Exchange-Traded Fund Risk: The risks of owning an ETF generally reflect the risks of owning the securities in which the ETF invests or is designed to track, although lack of liquidity in an ETF could result in it being more volatile than its underlying portfolio securities. Disruptions in the markets for the securities underlying ETFs purchased or sold by the Fund could result in losses on the Fund's investment in ETFs. ETFs also have management fees and transaction costs that may make them more expensive than owning the underlying securities directly.

Past Performance

Class T shares have not commenced operations as of the date of this Prospectus. As a result, the following chart and table give you a picture of the performance for the other share classes of the Fund, which are not offered in this Prospectus. The performance of Class T shares would be similar to the other share classes because all share classes are invested in the same portfolio of securities and would differ only to the extent that Class T shares have different expenses. To the extent Class T shares have higher expenses than the other share classes of the Fund, its performance would be lower.

The following bar chart and table 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 last ten years. Sales loads, if any, 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-, five- and ten-year periods compare to those of two broad-based securities market indices. The Fund has selected the MSCI ACWI^® (All Country World Index) Ex U.S.as its primary benchmark. The MSCI ACWI^® Ex U.S. is a free float-adjusted market capitalization weighted index that is designed to measure the equity market performance of developed and emerging markets, excluding the U.S. The Fund has selected the MSCI EAFE^® Index as secondary benchmark. The MSCI EAFE^® Index consists of international stocks representing the developed world outside of North America.

Performance data for the classes varies based on differences in their fee and expense structures. Performance is not shown for classes with less than one calendar year of performance. Past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Please visit nylinvestments.com/funds for more recent performance information.

The Fund's subadvisor changed effective January 1, 2018 due to an organizational restructuring whereby all investment personnel of Cornerstone Capital Management Holdings LLC, the former subadvisor, transitioned to MacKay Shields LLC.

Annual Returns, Class B Shares

(by calendar year 2008-2017)

Best Quarter
2Q/09	16.92	%
Worst Quarter
3Q/11	-19.57	%

Average Annual Total Returns (for the periods ended December 31, 2017)

Average Annual Total Returns - - MainStay MacKay International Equity Fund	1 Year	5 Years	10 Years	Inception Date
Class B	25.68%	6.40%	1.82%	Sep. 13, 1994
Class B | After Taxes on Distributions	25.68%	6.52%	1.71%
Class B | After Taxes on Distributions and Sales	14.54%	5.13%	1.53%
Class A	24.82%	6.66%	2.34%	Jan. 03, 1995
Investor Class	24.33%	6.29%	2.62%	Feb. 28, 2008
Class C	29.68%	6.69%	1.82%	Sep. 01, 1998
Class I	32.34%	8.13%	3.19%	Jan. 02, 2004
Class R1	32.31%	8.04%	3.10%	Jan. 02, 2004
Class R2	31.94%	7.77%	2.84%	Jan. 02, 2004
Class R3	31.59%	7.48%	2.58%	Apr. 28, 2006
MSCI ACWI® Ex U.S. Index (reflects no deductions for fees, expenses, or taxes)	27.19%	6.80%	1.84%
MSCI EAFE® Index (reflects no deductions for fees, expenses, or taxes)	25.03%	7.90%	1.94%

After-tax returns are calculated using the highest individual federal marginal income tax rates in effect at the time of each distribution or capital gain or upon the sale of fund shares, 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 B shares. After-tax returns for the other share classes may vary.

[1] The management fee is as follows: 0.89% on assets up to $500 million and 0.85% on assets over $500 million.