The S&P 500 fell 20.47% on October 19, 1987, according to its official closing level.
October 19, 1987, is known as Black Monday. The S&P 500 experienced its largest one-day percentage decline in its history, while stock markets in other countries also suffered severe losses. The Dow Jones Industrial Average fell 22.6 percent that day, a figure often cited in summaries of the crash.
The causes included high valuations, rising interest rates, trade tensions, and portfolio-insurance strategies that generated additional selling as prices declined. The precise balance among these factors remains a subject of financial research.
After the crash, regulators developed circuit breakers and other mechanisms designed to slow trading during extreme moves. The market recovered over time, and the crash did not produce a depression comparable to the 1930s. The S&P 500 measures large U.S. companies and differs from the narrower 30-stock Dow.