The authors conducted a time-series analysis to estimate the acute effects of high temperature in 6 cities in Korea and to compare thresholds of temperature on daily mortality among the cities. They examined the association between total mortality and the daily mean temperature and heat index during the summers in Korea from 1994 to 2003. The threshold temperature was estimated to be between 27.0 degrees C and 29.7 degrees C for 4 cities. For a daily mean temperature increase of 1 degrees C above the thresholds in Seoul, Daegu, Incheon, and Gwangju, estimated percentage increases in daily mortality were 16.3 (95% confidence interval [CI] = 14.2-18.4), 9.10 (CI = 5.12-13.2), 7.01 (CI = 4.42-9.66), and 6.73 (CI = 2.47-11.2), respectively. These city-specific threshold temperatures and the magnitude of the effects of hot temperature indicate that any analysis of the impact of climate change should take into account regional differences.