Which stock-market index fell 22.6% on October 19, 1987, setting its largest one-day percentage loss?

The Dow Jones Industrial Average fell 22.6% on October 19, 1987, setting its largest one-day percentage loss. The day became known as Black Monday and remains one of the most dramatic sessions in modern market history.

The decline followed sharp losses in several international markets, including Hong Kong, London, and Australia. In the United States, computerized trading, portfolio insurance, investor anxiety, and heavy selling contributed to the speed of the fall. The Dow lost 508 points, a record point decline at the time, although percentage comparisons are more meaningful across different eras.

Black Monday did not immediately produce a prolonged economic depression. The Federal Reserve supplied liquidity, and markets eventually stabilized. The event nevertheless led exchanges and regulators to examine trading systems and introduce circuit breakers designed to pause trading during extreme declines.

A common mix-up is calling the S&P 500 the index that lost 22.6%. The S&P 500 also fell sharply, but the famous 22.6% figure refers to the Dow’s percentage drop.