Which index suffered the biggest one-day percentage fall in its history on October 19, 1987?

The Dow Jones Industrial Average suffered its biggest one-day percentage fall on October 19, 1987.

On Black Monday, the Dow Jones Industrial Average fell 22.6 percent in a single trading session. The decline was the largest one-day percentage drop in the index’s history and became the defining statistic of the 1987 stock-market crash.

Several factors contributed to the panic, including high valuations, concerns about economic conditions, international trade tensions, and automated portfolio-insurance trading. Selling spread rapidly across global markets, although the causes and effects varied by country.

The 1987 crash is often confused with the 1929 crash because both produced famous “Black” trading days. Black Monday in 1987 was a single-day international market shock, while the 1929 collapse developed through several major sessions and was followed by the much broader Great Depression. The Federal Reserve helped calm markets by signaling that it would provide liquidity to the financial system.