MainStay Winslow Large Cap Growth Fund Formerly known as MainStay Large Cap Growth Fund		Summary Prospectus August 31, 2020
Class/Ticker	SIMPLE   MLRMX

Before you invest, you may want to review the Fund's Prospectus, which contains more information about the Fund and its risks. You can find the Fund's Prospectus, reports to shareholders and other information about the Fund by going online to nylinvestments.com/documents, by calling 800-624-6782 or by sending an e-mail to MainStayShareholderServices@nylim.com. The Fund's Prospectus and Statement of Additional Information, both dated August 31, 2020, as may be amended from time to time, are incorporated by reference into this Summary Prospectus.

Beginning on January 1, 2021, paper copies of the Fund’s annual and semiannual shareholder reports will no longer be sent by mail, unless you specifically request paper copies of the reports from MainStay Funds or from your financial intermediary. Instead, the reports will be made available on the MainStay Funds’ website. You will be notified by mail and provided with a website address to access the report each time a new report is posted to the website.

If you already elected to receive shareholder reports electronically, you will not be affected by this change and you need not take any action. At any time, you may elect to receive reports and other communications from MainStay Funds electronically by calling toll-free 800-624-6782, by sending an e-mail to MainStayShareholderServices@nylim.com, or by contacting your financial intermediary.

You may elect to receive all future shareholder reports in paper form free of charge. If you hold shares of the Fund directly, you can inform MainStay Funds that you wish to receive paper copies of reports by calling toll-free 800-624-6782 or by sending an e-mail to MainStayShareholderServices@nylim.com. If you hold shares of the Fund through a financial intermediary, please contact the financial intermediary to make this election. Your election to receive reports in paper form will apply to all MainStay Funds in which you are invested and may apply to all funds held with your financial intermediary.

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 be required to pay a commission or other transaction charge to your financial intermediary for effecting transactions in a class of shares of the Fund that has no initial sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution, such as SIMPLE Class. These commissions are not reflected in the fee and expense table or expense example below.

		SIMPLE Class
Shareholder Fees (fees paid directly from your investment)
	Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	None
	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)
	Management Fees (as an annual percentage of the Fund's average daily net assets)1	0.62	%
	Distribution and/or Service (12b-1) Fees	0.50	%
	Other Expenses2	0.23	%
	Total Annual Fund Operating Expenses	1.35	%

1.The management fee is as follows: 0.75% on assets up to $500 million; 0.725% on assets from $500 million to $750 million; 0.71% on assets from $750 million to $1 billion; 0.70% on assets from $1 billion to $2 billion; 0.66% on assets from $2 billion to $3 billion; 0.61% on assets from $3 billion to $7 billion; 0.585% on assets from $7 billion to $9 billion; and 0.575% on assets over $9 billion.

2.Based on amounts expected to be incurred during the current fiscal year.

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:

Expenses After	SIMPLE
	Class
1 Year	$ 137
3 Years	$ 428
5 Years	$ 739
10 Years	$1,624

MSLG01SI-08/20

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 54% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 80% of its assets (net assets plus any borrowings for investment purposes) in large capitalization companies, which are companies having a market capitalization in excess of $4 billion at the time of purchase. Typically, Winslow Capital Management, LLC, the Fund's Subadvisor, invests substantially all of the Fund's investable assets in domestic securities. However, the Fund is permitted to invest up to 20% of its net assets in foreign securities, which are generally securities issued by companies organized outside the United States or that trade primarily in non-U.S. securities markets.

Investment Process: The Fund invests in those companies that the Subadvisor believes will provide an opportunity for achieving superior portfolio returns (i.e., returns in excess of the returns of the average stock mutual fund) over the long term. The Subadvisor seeks to invest in companies that have the potential for above-average future earnings and cash flow growth with management focused on shareholder value.

When purchasing stocks for the Fund, the Subadvisor looks for companies typically having some or all of the following attributes: addressing markets with growth opportunities; leads or gains in market share; identifiable and sustainable competitive advantages; managed by a team that can perpetuate the firm's competitive advantages; high, and preferably rising, returns on invested capital; deploys excess cash flow to enhance shareholder return; and demonstrates sound corporate governance. The Subadvisor may give consideration to certain environmental, social, and governance (“ESG”) criteria when evaluating an investment opportunity.

The Subadvisor takes a "bottom-up" investment approach when selecting investments. This means it bases investment decisions on company specific factors, not general economic conditions.

Under normal market conditions, the Subadvisor employs a sell discipline pursuant to which it may sell some or all of its position in a stock when a stock becomes fully valued, the fundamental business prospects are deteriorating, or the position exceeds limits set by the Subadvisor.

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 Subadvisor may underperform the market in which the Fund invests 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. Such changes 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 analyses used by the Subadvisor may not produce the desired results. The Subadvisor may give consideration to certain ESG criteria when evaluating an investment opportunity. The application of ESG criteria may result in the Fund (i) having exposure to certain securities or industry sectors that are significantly different than the composition of the Fund's benchmark; and (ii) performing differently than other funds and strategies in its peer group that do not take into account ESG criteria or the Fund's benchmark.

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.

Market Capitalization Risk: To the extent the Fund invests in securities issued by small-, mid-, or large-cap companies, the Fund will be subject to the risks associated with securities issued by companies of the applicable market capitalization.  Securities of small-cap and mid-cap companies may be subject to greater price volatility, significantly lower trading volumes, cyclical, static or moderate growth prospects and greater spreads between their bid and ask prices than securities of larger companies. Smaller capitalization companies frequently rely on narrower product lines and niche markets and may be more vulnerable to adverse business or market developments.  Securities issued by larger companies may have less growth potential and may not be able to attain the high growth rates of successful smaller companies, especially during strong economic periods.  In addition, larger companies may be less capable of responding quickly to competitive challenges and industry changes, including those resulting from improvements in technology, and may suffer sharper price declines as a result of earnings disappointments.  There is a risk that the securities issued by companies of a certain market capitalization may underperform the broader market at any given time.

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 (non-U.S.) securities may be riskier than investments in U.S. securities. Foreign regulatory regimes and securities markets can have less stringent investor protections and disclosure standards and less liquid trading markets than U.S. regulatory regimes and securities markets, and can experience political, social and economic developments that 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

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an investment increase or decrease, unrelated to the quality or performance of the investment itself. These risks may be greater with respect to securities of companies that conduct their business activities in emerging markets or whose securities are traded principally in emerging markets.

Past Performance

SIMPLE Class shares have not commenced operations as of the date of this Prospectus. The following chart and table reflect the performance for the other share classes of the Fund, which are not offered in this Prospectus. The performance of SIMPLE Class 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 SIMPLE Class shares have different expenses. Because SIMPLE Class shares have higher expenses than all other share classes of the Fund, except Class B and Class C shares, the performance of SIMPLE Class shares would be lower than that of the other share classes, except Class B and Class C shares.

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 time. 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) 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^® Index companies with higher price-to-book ratios and higher forecasted growth values. The Fund has selected the Standard & Poor’s 500^® Index (“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 data for the classes varies based on differences in their fee and expense structures. Performance data 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.

Annual Returns, Class I Shares

(by calendar year 2010-2019)

Best Quarter
1Q/19	17.58	%
Worst Quarter
4Q/18	-15.98	%

As of June 30, 2020, the Class I shares of the Fund had a year to date return of 12.22%.

Average Annual Total Returns (for the periods ended December 31, 2019)

						10 Years or
	Inception	1 Year		5 Years		Since
						Inception
Return Before Taxes
Class I	4/1/2005	33.67	%	13.75	%	14.19	%
Return After Taxes on Distributions
Class I		30.54	%	10.30	%	12.02	%
Return After Taxes on Distributions and Sale of Fund Shares
Class I		22.00	%	10.22	%	11.37	%
Return Before Taxes
Class A	7/1/1995	26.06	%	12.20	%	13.24	%
Investor Class	2/28/2008	25.83	%	12.07	%	13.15	%
Class B	4/1/2005	27.07	%	12.32	%	12.96	%
Class C	4/1/2005	31.18	%	12.51	%	12.95	%
Class R1	4/1/2005	33.46	%	13.64	%	14.05	%
Class R2	4/1/2005	33.16	%	13.33	%	13.78	%
Class R3	4/28/2006	32.82	%	13.06	%	13.48	%
Class R6	6/17/2013	33.87	%	13.87	%	15.51	%
Russell 1000® Growth Index (reflects no deductions for fees, expenses, or taxes)		36.39	%	14.63	%	15.22	%
S&P 500® Index (reflects no deductions for fees, expenses, or taxes)		31.49	%	11.70	%	13.56	%

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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 I shares. After-tax returns for the other share classes may vary.

Management

New York Life Investment Management LLC serves as the Fund's Manager. Winslow Capital Management, LLC serves as the Subadvisor. The individuals listed below are jointly and primarily responsible for the day-to-day portfolio management of the Fund.

Subadvisor	Portfolio Managers	Fund Service Date
Winslow Capital Management, LLC	Justin H. Kelly, Chief Executive Officer & Chief Investment Officer	Since 2005
	Patrick M. Burton, Senior Managing Director	Since 2013

How to Purchase and Sell Shares

You may purchase or sell shares of the Fund on any day the Fund is open for business by contacting your financial adviser or financial intermediary firm, or by contacting the Fund by telephone at 800-624-6782, by mail at MainStay Funds, P.O. Box 219003, Kansas City, MO 64121-9000, by overnight mail to 430 West 7^th Street, Suite 219003, Kansas City, MO 64105-1407, or by accessing our website at nylinvestments.com/funds. SIMPLE Class shares are generally only available to SIMPLE IRA Plan accounts. SIMPLE Class shares are generally not available to retail accounts. Generally, an initial investment minimum of $1,000 applies for SIMPLE Class shares. SIMPLE Class shares have no subsequent investment minimum.

Certain financial intermediaries through whom you may invest may impose their own investment minimums, fees, policies and procedures for purchasing and selling Fund shares, which are not described in this Prospectus or the Statement of Additional Information, and which will depend on the policies, procedures and trading platforms of the financial intermediary. Consult a representative of your financial intermediary about the availability of shares of the Fund and the intermediary's policies, procedures and other information.

Tax Information

The Fund's distributions are generally taxable to you as ordinary income, capital gains, or a combination of the two, unless you are investing through a tax-deferred arrangement, such as a 401(k) plan or an individual retirement account.

Compensation to Financial Intermediary Firms

If you purchase Fund shares through a financial intermediary firm (such as a broker/dealer or bank), the Fund and its related companies may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the financial intermediary firm or your financial adviser to recommend the Fund over another investment. Ask your financial adviser or visit your financial intermediary firm's website for more information.

"New York Life Investments” is both a service mark, and the common trade name, of certain investment advisors affiliated with New York Life Insurance Company.

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