The largest single-day percentage fall in Hong Kong’s Hang Seng Index during the 1997 Asian crisis occurred on 28 October 1997.
The Hang Seng fell 13.7% that day, amid intense pressure on Asian currencies and financial markets. Investors were reassessing the region after Thailand abandoned its currency peg in July and the crisis spread to Indonesia, South Korea, and other economies.
Hong Kong maintained its linked exchange-rate system, but defending the link required very high interest rates and created severe stress in local financial markets. The plunge also reflected fears that Hong Kong’s economy and property market would be hit by regional contagion.
The crisis was not limited to stock prices. Currency devaluations, foreign-currency borrowing, banking weaknesses, and large capital outflows all contributed. Hong Kong’s market later recovered, but the episode became a vivid example of how a currency defense can coincide with a sharp equity-market sell-off.