The S&P 500 lost 20.47% on Black Monday, October 19, 1987.
That one-day percentage decline remains the largest daily loss in the index’s history. Selling spread across international markets, and the Dow Jones Industrial Average fell 22.6%, its largest one-day percentage drop. Unlike the 1929 crash, the 1987 plunge was not followed by a comparable Great Depression, although it caused severe concern about market stability.
Several factors contributed to the collapse. Valuations had risen, investors were worried about interest rates and trade tensions, and computerized portfolio-insurance strategies generated additional sell orders as prices fell. These mechanisms could reinforce downward momentum in a market with limited liquidity.
The crash led exchanges and regulators to develop new protections. U.S. markets later adopted circuit breakers designed to pause trading after unusually large declines. The 20.47% figure refers specifically to the S&P 500’s closing-to-closing loss, while the Dow’s often-quoted 22.6% figure describes a different index.