MainStay MacKay Small Cap Core Fund

MainStay MacKay Small Cap Core Fund (formerly known as MainStay Epoch U.S. Small Cap 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 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.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees	MainStay MacKay Small Cap Core Fund SIMPLE Class
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)

Annual Fund Operating Expenses	MainStay MacKay Small Cap Core Fund SIMPLE Class
Management Fees (as an annual percentage of the Fund's average daily net assets)	0.80%	[1]
Distribution and/or Service (12b-1) Fees	0.50%
Other Expenses	0.57%	[2]
Total Annual Fund Operating Expenses	1.87%
Waivers / Reimbursements	(0.13%)	[3]
Total Annual Fund Operating Expenses After Waivers / Reimbursements	1.74%	[3]

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	MainStay MacKay Small Cap Core Fund SIMPLE Class USD ($)
1 Year	$ 177
3 Years	575
5 Years	999
10 Years	$ 2,180

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 205% of the average value of its portfolio.

Principal Investment Strategies

The Fund invests, under normal circumstances, at least 80% of its assets (net assets plus any borrowings for investment purposes) in companies with market capitalizations at the time of investment that are similar to the market capitalizations of companies in the Russell 2000^® Index, and invests primarily in common stocks of U.S. companies. As of June 30, 2020, companies in the Russell 2000^® Index had market capitalizations ranging from $13 million to $8.34 billion.

Investment Process: MacKay Shields LLC, the Fund's Subadvisor, seeks to construct a broadly diversified portfolio across sectors and industries using quantitative analysis to identify undervalued and overvalued securities. The Subadvisor uses a quantitative model that is designed to evaluate individual issuers and securities across multiple criteria, including valuation, momentum and market sentiment. The Subadvisor also conducts a qualitative analysis to account for events and conditions that may not be quantifiable by the analysis, such as company-specific and market events. The Subadvisor evaluates the quantitative model and, from time to time, the Subadvisor may adjust the metrics and data underlying its quantitative analysis or model for a variety of reasons, including, without limitation, to account for changing market, financial or economic conditions. Using an objective, disciplined and broadly-applied process, the Subadvisor selects securities that it believes have the most potential to appreciate, while seeking to limit exposure to risk. The Subadvisor also seeks to control the Fund’s exposure to risk by diversifying the Fund’s investments over securities issued by a large number of companies.

The Subadvisor may sell a security if, among other reasons, it believes the security will no longer contribute to meeting the investment objective of the Fund, if better opportunities are identified, or if it determines the initial investment expectations are not being met.

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. Investments selected using quantitative methods or based on models that analyze information and data (“quantitative tools”) may perform differently from the market as a whole. The quantitative tool used by the Subadvisor, and the investments selected based on the quantitative tool, may not perform as expected. The quantitative tool may contain certain assumptions in construction and implementation that may adversely affect the Fund’s performance. There may also be technical issues with the construction and implementation of quantitative tools (for example, software or other technology malfunctions, or programming inaccuracies). In addition, the Fund’s performance will reflect, in part, the Subadvisor’s ability to make active qualitative decisions and timely adjust the quantitative tool, including the tool’s underlying metrics and data.

Small-Cap and Mid-Cap Stock Risk: The general risks associated with equity securities and liquidity risk are particularly pronounced for stocks of companies with market capitalizations that are small compared to other publicly traded companies. These companies may have limited product lines, markets or financial resources or they may depend on a few key employees. Stocks of small-capitalization and mid-capitalization companies may trade less frequently and in lesser volume than more widely held securities, and their values may fluctuate more sharply than those of other securities. They may also trade in the over-the-counter market or on a regional exchange, or may otherwise have limited liquidity. Generally, the smaller the company, the greater these risks become.

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.

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.

Value Stock Risk: Value stocks may never reach what the Subadvisor believes is their full value or they may go down in value. In addition, different types of stocks tend to shift in and out of favor depending on market and economic conditions, and therefore the Fund's performance may be lower or higher than that of funds that invest in other types of equity securities.

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 a broad-based securities market index. The Fund has selected the Russell 2000^® Indexas its primary benchmark as a replacement for the Russell 2500^TM Index because it believes that the Russell 2000^® Index is more reflective of its current investment style. The Russell 2000^® Index measures the performance of the small-cap segment of the U.S. equity universe. It is a subset of the Russell 3000^® Index and includes approximately 2,000 of the smallest securities based on a combination of their market cap and current index membership. The Russell 2500^TM Index measures the performance of the small to mid-cap segment of the U.S. equity universe, commonly referred to as "smid" cap. The Russell 2500^TM Index is a subset of the Russell 3000^® Index. It includes approximately 2,500 of the smallest securities based on a combination of their market cap and current index membership.

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.

The Fund replaced its subadvisor effective April 1, 2019, and changed its investment objective and principal investment strategies. The past performance in the bar chart and table prior to that date reflects the Fund's prior subadvisor, investment objective and principal investment strategies.

Annual Returns, Class I Shares (by calendar year 2010-2019)

Best Quarter
3Q/10	15.16	%
Worst Quarter
3Q/11	-19.84	%

As of June 30, 2020, the Class I shares of the Fund had a year to date return of -17.40%.

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

Average Annual Returns - MainStay MacKay Small Cap Core Fund	1 Year	5 Years or Since Inception	10 Years or Since Inception	Inception Date
Class I	17.69%	4.87%	9.74%	Jan. 12, 1987
Class A	10.88%	3.43%	8.84%	Jan. 02, 2004
Investor Class	10.63%	3.16%	8.55%	Feb. 28, 2008
Class B	11.14%	3.23%	8.35%	Jan. 02, 2004
Class C	15.21%	3.55%	8.35%	Dec. 30, 2002
Class R1	17.56%	4.77%	9.92%	Jul. 31, 2012
Class R2	17.26%	4.51%	9.64%	Jul. 31, 2012
Class R3	16.97%	8.55%		Feb. 29, 2016
After Taxes on Distributions | Class I	17.55%	3.60%	9.04%
After Taxes on Distributions and Sale of Fund Shares | Class I	10.57%	3.66%	7.96%
Russell 2000® Index (reflects no deductions for fees, expenses, or taxes)	25.52%	8.23%	11.83%
Russell 2500® Index (reflects no deductions for fees, expenses, or taxes)	27.77%	8.93%	12.58%

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.

[1] The management fee is as follows: 0.80% on assets up to $1 billion; 0.775% on assets from $1 billion to $2 billion; and 0.75% on assets over $2 billion.

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

[3] New York Life Investment Management LLC ("New York Life Investments") has contractually agreed to limit the transfer agency expenses charged to each of the Fund's share classes to a maximum of 0.35% of that share class's average daily net assets on an annual basis after deducting any applicable Fund or class-level expense reimbursements or small account fees. This agreement will remain in effect until August 31, 2021, and shall renew automatically for one-year terms unless New York Life Investments provides written notice of termination prior to the start of the next term or upon approval of the Board of Trustees of the Fund.