We analysed the concentration of radioactive cesium in dried Lentinula edodes (Shiitake) produced in Japan, and evaluated its use as an indicator of radioactive contamination. We collected Lentinula edodes (Shiitake) mushrooms grown on wood in 38 prefectures from north to south, and measured 137Cs, 134Cs and 40K concentrations with high purity Ge-detectors. The concentration ranges were as follows: 137Cs <0.003-61.1 Bq/kg, 40K 378.7-667.3 Bq/kg; 134Cs was undetectable. There was a positive correlation between 137Cs concentrations in Shiitake and those in rain and dry fallout (P < 0.05). They were also correlated inversely with atmospheric temperature (P < 0.01). The multiple correlation coefficient comparing 137Cs concentration with atmospheric temperatures, 40K concentrations and fallout levels was 0.56 which was statistically significant (P < 0.01). A variance analysis based on the 137Cs concentration classified by geographical area (Hokkaido-Tohoku, Kanto, Chubu, Kinki, Chugoku, Shikoku and Kyushu) showed that the variance between the groups was significantly greater than that within each group which, together with the above-mentioned information, suggests that the 137Cs levels are affected primarily by these environmental factors.