An operational procedure for alerting the public of stressful weather (exceptionally hot and cold situations) in Hong Kong has been implemented since late 1997. From a 28-year (1968-1995) climatological database, criteria based on a weather stress index (WSI) was established to define stressful weather conditions. A special bulletin is issued to the public when exceptionally high (hot, i.e. ≥ 97.5%) or low (cold, i.e. ≤ 2.5%) WSI is forecast. The bulletin provides information on current or predicted stressful weather conditions as well as statements on precautionary action to take. A study was also carried out to examine the effects of WSI on mortality rates on days with extreme WSI. Preliminary results indicate that there was a relationship between low WSI and mortality rates in winter. In summer, however, the results were not conclusive.