The S&P 500 fell 20.47% on October 19, 1987, during Black Monday.
That decline was part of a synchronized global crash. The Dow Jones Industrial Average fell 22.6%, while the S&P 500, which tracks large U.S. companies across many industries, also suffered an unprecedented one-day percentage loss.
The crash reflected both economic concerns and market-structure problems. Investors worried about interest rates, trade imbalances, and high valuations. Computerized trading systems and portfolio insurance encouraged further selling once prices began to fall, creating feedback that intensified the decline.
The S&P 500 is often confused with the Dow because both are widely quoted U.S. benchmarks. The Dow contains 30 prominent companies and is price-weighted. The S&P 500 contains about 500 large companies and is weighted by market capitalization, making it a broader measure of the U.S. equity market.