Which U.S. stock exchange suffered the largest percentage one-day loss in its history on October 19, 1987?

The New York Stock Exchange suffered its largest percentage one-day loss in history on October 19, 1987.

That day, known as Black Monday, the Dow Jones Industrial Average fell 22.6%, while markets around the world also dropped sharply. The New York Stock Exchange was the principal U.S. marketplace where the historic selling occurred, although the crash affected multiple exchanges and financial instruments.

Several factors contributed, including high share valuations, investor concern about interest rates and trade imbalances, portfolio insurance strategies, and rapidly increasing computer-assisted trading. No single cause fully explains the collapse. Regulators later introduced or strengthened circuit breakers designed to pause trading during extreme declines. Black Monday is distinct from the 1929 crash and from the 2010 Flash Crash: the dates, market conditions, and immediate mechanisms were different.