The largest one-day percentage drop in Dow Jones history was 22.6%, recorded during the 1987 crash.
On October 19, 1987, known as Black Monday, the Dow Jones Industrial Average fell 508 points, equivalent to 22.6% of its previous closing value. The decline spread across international markets, following sharp losses in Hong Kong, Europe, and elsewhere. It remains the Dow's greatest single-day percentage fall, even though later crashes produced larger point losses because the index had grown much higher.
Several forces contributed to the panic, including stretched valuations, rising interest rates, market uncertainty, and computerized portfolio-insurance strategies. Those automated sell programs could amplify downward momentum when prices fell. The precise causes remain debated, but the scale of the synchronized selling is undisputed.
The 1987 crash differed from 1929 because it was not followed by a comparable long depression. Federal Reserve Chair Alan Greenspan issued a widely cited reassurance, and exchanges later introduced circuit breakers to slow trading during extreme moves.