The Dow Jones Industrial Average suffered its largest one-day percentage loss on 19 October 1987.
That Monday, commonly known as Black Monday, the Dow fell 508 points, or 22.6%, in a single trading session. The percentage decline remains the largest one-day loss in the index’s history. Selling spread rapidly through international markets, so the event was not confined to the United States.
Several factors were associated with the crash, including high valuations, concerns about interest rates and trade deficits, and computer-driven portfolio-insurance strategies. The exact cause is still discussed by financial historians, but the scale of the decline is not in dispute. The crash also led to the development of market-wide circuit breakers, designed to pause trading during exceptionally rapid falls.