Hong Kong’s Hang Seng Index fell about 45.8% during the 1987 Black Monday crash.
The Hong Kong market experienced one of the world’s most severe declines during the international sell-off. Trading was suspended for several days after the collapse, and the exchange reopened on 26 October 1987. The Hang Seng then suffered another major fall, intensifying losses across the region.
The shock was global rather than limited to the United States. Concerns about valuations, interest rates, currency conditions, and automated portfolio-insurance selling contributed to synchronized declines. Markets in Europe, North America, Australia, and Asia all recorded dramatic losses.
The 45.8% figure is associated with the Hang Seng’s fall during the crash period, not with the Dow Jones Industrial Average’s separate 22.6% one-day decline on 19 October. Those percentages describe different markets and measurement periods.