Which U.S. trading day saw the largest one-day percentage fall in Dow Jones history?

19 October 1987 saw the largest one-day percentage fall in Dow Jones Industrial Average history.

Often called Black Monday, the trading day saw the Dow decline 22.6%, or 508 points. The fall exceeded the percentage losses recorded during the worst days of the 1929 Wall Street crash, even though the Dow’s point total was smaller because the index level was much lower.

The decline occurred after a prolonged bull market and amid concerns about interest rates, currency values, trade tensions, and market valuation. Computerized trading and portfolio-insurance techniques may have accelerated selling, although historians and economists continue to debate the relative importance of each factor.

The statistic concerns the percentage change, not the largest point loss. That distinction matters because index levels change over time: a later market day can lose more points while still posting a smaller percentage decline.