Which U.S. stock index suffered the largest one-day percentage loss during Black Monday in 1987?

During Black Monday in 1987, the Dow Jones Industrial Average suffered the largest one-day percentage loss among the major U.S. stock indexes, falling 22.6 percent.

The decline occurred on October 19, 1987, when the Dow dropped 508 points. The fall was especially striking because it exceeded the percentage loss recorded on any single day during the 1929 Wall Street crash. Trading volume was also extraordinarily high for the period.

The S&P 500 and Nasdaq Composite also plunged, so the crash was broad rather than limited to industrial companies. However, the Dow’s 22.6 percent fall remains the standard headline statistic associated with Black Monday.

The event led regulators and exchanges to develop coordinated trading halts. These circuit breakers were intended to give investors time to absorb information and reduce the risk of automatic selling feeding on itself.