The Dow Jones Industrial Average’s largest one-day percentage loss was 22.6% on Black Monday, October 19, 1987.
The Dow fell 508 points during that session, a historically enormous decline relative to the index’s level. Selling spread rapidly across international markets, making the 1987 crash a genuinely global event rather than a disturbance confined to the United States.
Several factors were associated with the crash, including high share valuations, rising interest rates, trade tensions, and concerns about the U.S. dollar. Computerized portfolio-insurance strategies and other program-trading systems also accelerated selling once prices began to fall.
The 22.6% figure is a percentage decline, not the number of index points lost. That distinction matters because the Dow’s point loss depends on its starting level. The market later recovered, and the crash did not produce an economic depression comparable to the Great Depression.