The Dow Jones Industrial Average lost 22.6% on Black Monday, October 19, 1987.
The fall was extraordinary because it happened during a single trading session. The Dow dropped 508 points, and the percentage loss was calculated from its previous closing level. The crash also spread across major international markets, including Hong Kong, Australia, Britain, and Canada, although market declines varied by country and date.
The episode exposed weaknesses in trading systems and in investment strategies that automatically sold when prices moved downward. A falling market could therefore trigger further selling without a new piece of economic news explaining every transaction. Investigations also examined liquidity problems and the growing use of computer-assisted trading.
The 22.6% figure is often compared with the 12.8% decline on Black Monday in 1929, but the two dates are separated by 58 years. Neither number describes the largest percentage fall in every national index; it specifically identifies the Dow's 1987 one-day loss.