The Dow Jones Industrial Average fell 22.6% on October 19, 1987, the largest one-day percentage decline in its history.
The date became known as Black Monday. Selling spread through markets in the United States and abroad, and the Dow lost 508 points. Program trading, high valuations, concerns about interest rates, and market-liquidity problems all contributed to the panic, although historians do not identify one single cause.
The decline was unusually rapid because modern trading systems and portfolio strategies could generate large sell orders in a short time. The crash also exposed weaknesses in how markets handled heavy order volumes.
The loss is measured by percentage, not points. Later market days produced larger point declines because the Dow's numerical level had grown substantially, but none matched Black Monday's percentage fall.