On October 19, 1987, which U.S. stock index suffered its largest one-day percentage drop, falling 22.6%?

On October 19, 1987, the Dow Jones Industrial Average suffered its largest one-day percentage drop, falling 22.6%.

The collapse became known as Black Monday. It affected markets worldwide, with major exchanges in Europe, Asia, Australia, and North America recording severe losses. The Dow’s fall was especially striking because it exceeded the percentage decline seen on any single day during the 1929 crash.

Several explanations have been proposed, including computerized program trading, portfolio insurance, market overvaluation, rising interest rates, and international financial tensions. No single cause completely accounts for the speed and scale of the selling.

The crash led regulators to introduce coordinated trading halts, commonly called circuit breakers. A frequent mistake is to confuse the Dow’s 22.6% decline with the S&P 500’s smaller percentage loss on the same day; the famous record belongs specifically to the Dow.