Which U.S. stock index lost nearly 23% on Black Monday in 1987?

The Dow Jones Industrial Average lost nearly 23% on Black Monday in 1987.

On 19 October 1987, the Dow fell 508 points, equivalent to 22.6% of its value at the start of the session. The drop was extraordinary because it occurred in one trading day, not over a prolonged bear market. It remains the Dow’s largest one-day percentage decline.

The crash was part of a worldwide market sell-off. Hong Kong’s market fell first, and declines spread through Europe before reaching the United States. In America, program trading and portfolio-insurance systems automatically generated additional sell orders as prices declined.

The S&P 500 and Nasdaq also suffered severe losses, so the Dow was not the only important index affected. However, the Dow’s long history and prominent media coverage made its fall the most widely remembered numerical measure of the crash.