Which U.S. index lost about 20% in one day during the 1987 Black Monday crash?

The Dow Jones Industrial Average lost about 20% in one day during the 1987 Black Monday crash.

On October 19, 1987, the Dow fell 508 points, a decline of 22.6%, then the largest one-day percentage drop in the index’s history. Markets in other countries also suffered severe losses, making the event a global stock-market crash rather than an isolated New York episode.

Several factors were discussed afterward, including high valuations, rising interest rates, concerns about trade deficits, and computerized portfolio-insurance strategies. No single explanation completely accounts for the worldwide speed and scale of the decline.

The percentage loss is often confused with the 508-point loss. Points depend on the index’s level and therefore are not directly comparable across decades; the percentage figure better expresses the day’s severity. The crash led to market-structure reforms, including coordinated trading halts and other circuit-breaker mechanisms.