BACKGROUND: Although age and regional climate are considered to have effects on the incidence ratio of heat-related illness, quantitative estimation of age or region on the effect of occurring temperature for heat stroke is limited. METHODS: By utilizing data on the number of daily heat-related ambulance transport (HAT) in each of three age groups (7-17, 18-64, 65 years old, or older) and 47 prefectures in Japan, and daily maximum temperature (DMT) or Wet Bulb Globe Temperature (DMW) of each prefecture for the summer season, the effects of age and region on heat-related illness were studied. Two-way ANOVA was used to analyze the significance of the effect of age and 10 regions in Japan on HAT. The population-weighted average of DMT or DMW measured at weather stations in each prefecture was used as DMT or DMW for each prefecture. DMT or DMW when HAT is one in 100,000 people (T(1) and W(1), respectively) was calculated for each age category and prefecture as an indicator of heat acclimatization. The relation between T(1) or W(1) and average DMT or DMW of each age category and prefecture were also analyzed. RESULTS: HAT of each age category and prefecture was plotted nearly on the exponential function of corresponding DMT or DMW. Average R(2) of the regression function in 47 prefectures in terms of DMW was 0.86, 0.93, and 0.94 for juveniles, adults, and elderly, respectively. The largest regional difference of W(1) in 47 prefectures was 4.5 and 4.8 °C for juveniles and adults, respectively between Hokkaido and Tokyo, 3.9 °C for elderly between Hokkaido and Okinawa. Estimated W(1) and average DMT or DMW during the summer season for 47 prefectures was linearly related. Regarding age difference, the regression line showed that W(1) of the prefecture for DMW at 30 °C of WBGT was 31.1 °C, 32.4 °C, and 29.8 °C for juveniles, adults, and elderly, respectively. CONCLUSIONS: Age and regional differences affected the incidence of HAT. Thus, it is recommended that public prevention measures for heat-related disorders take into consideration age and regional variability.