The S&P 500 declined 20.47 percent on Black Monday, October 19, 1987.
That fall was the largest one-day percentage loss in the S&P 500’s history. The index’s decline occurred alongside the Dow’s 22.6 percent plunge and severe losses in markets around the world. Investors faced concerns about valuations, interest rates, trade imbalances, and the growing use of computerized trading strategies.
The crash prompted regulators and exchanges to reconsider market structure. Circuit breakers, which temporarily halt trading after exceptionally large moves, were introduced in the United States after the episode and later revised.
The S&P 500 is broader than the Dow because it represents 500 large U.S. companies rather than 30. The two indexes therefore recorded different percentage losses on the same day. Point losses are also not directly comparable across decades because index levels change over time.