In order to describe the relationship between mortality rate and extreme heat during the summer of 1999 in Hokkaido, we calculated the monthly age-adjusted death rates, average monthly mean temperature and average monthly high temperature for the years 1995 to 1999 in Hokkaido.The materials were derived from Statistics and Information Department, Minister's Secretariat, Ministry of Health, Labour and Welfare of Japan, Statistical Bureau Management and Coordination Agency Government of Japan and the Japan Meteorological Agency.Trends in the monthly age-adjusted death rates and temperature changes in the same period were analyzed. The highest average monthly high temperature for August and September (28.8°C and 23.8°C, respectively) occurred in 1999;the similar trend was observed in the highest average monthly mean temperature.In August 1999, there were 14 days with highest temperatures of 30°C and over.The age-adjusted rate in August 1999 was significantly higher compared with those for the years 1995 to 1998 (p<0.01).We concluded that an unusually hot spell in 1999 was followed by a high mortality rate in Hokkaido.J Epidemiol, 2002;12:254-257