The S&P 500 fell 20.5% on October 19, 1987, setting a record one-day percentage decline for the index.
That session was part of the worldwide crash known as Black Monday. The S&P 500, which represents 500 leading U.S. companies, suffered a larger percentage drop than the Dow Jones Industrial Average's 22.6% decline only in the sense that the two figures describe different index histories and construction; the Dow's percentage fall was greater that day. The S&P's 20.5% loss remains its largest one-day percentage decline.
The crash followed a steep bull market and concerns about interest rates, international trade, and valuation. Program trading and portfolio-insurance strategies intensified selling as prices fell. In response, U.S. regulators developed market-wide circuit breakers. The S&P 500 is not the same as the Nasdaq Composite, which is more heavily associated with technology companies and experienced its defining crash during the dot-com bust.