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The classification of heat-related deaths solely according to body temperatures underestimates the magnitude of heat-related mortality during heat waves. Broader classifications are often used, but their utility in defining the mortality and identifying at-risk populations has not been evaluated. Using death data from the July 1995 heat wave in Chicago, the authors compared heat-related mortality rates based on the classification of heat-related deaths by the Cook County Medical Examiner's Office (CCMEO), with excess mortality rates based on total mortality differentials during and before the heat wave. In July 1995, the overall mortality in Chicago was 19 deaths per 100,000 population for heat-related mortality and 24 to 26 deaths per 100,000 population for excess mortality. Across Chicago community areas, the two mortality rates were closely related (r = 0.73-0.79; p < .01), but heat-related mortality rates were lower than excess mortality rates in community areas where excess mortality rates were higher (slope < 1; p < .01), a finding indicating an underestimation of heat-related deaths in such areas. The underestimation could not be explained by uncertainties in estimating excess mortality rates or by differences in socioeconomic and demographic characteristics among communities. These results support using the broader CCMEO classification of heat-related deaths as a relative indicator to target communities for prevention and relief efforts, but not as an adequate measure of actual heat-related mortality in a high-risk neighborhood.