We investigated the relationship between epidemics and soil radiation through an exploratory study using sentinel surveillance data (individuals aged <20 years) during the last three epidemic seasons of influenza and norovirus in Japan. We used a spatial analysis method of a geographical information system (GIS). We mapped the epidemic spreading patterns from sentinel incidence rates. We calculated the average soil radiation [dm (mu Gy/h)] for each sentinel site using data on uranium, thorium, and potassium oxide in the soil and examined the incidence rate in units of 0.01 mu Gy/h. The correlations between the incidence rate and the average soil radiation were assessed. Epidemic clusters of influenza and norovirus infections were observed in areas with relatively high radiation exposure. A positive correlation was detected between the average incidence rate and radiation dose, at r = 0.61-0.84 (P <0.01) for influenza infections and r = 0.61-0.72 (P <0.01) for norovirus infections. An increase in the incidence rate was found between areas with radiation exposure of 0 <dm <0.01 and 0.154dm <0.16, at 1.80 [95% confidence interval (CI) 1.47-2.12] times higher for influenza infection and 2.07 (95% CI 1.53-2.61) times higher for norovirus infection. Our results suggest a potential association between decreased immunity and irradiation because of soil radiation. Further studies on immunity in these epidemic-prone areas are desirable.