Geographic observation of the increased mortality of some cancers at higher latitudes has led to a hypothesis that vitamin D produced after exposure to solar radiation has anti-carcinogenic effects. However, it is unclear whether such association would be observed in countries like Japan, where fish consumption, and therefore dietary vitamin D intake, is high. Pearson correlation coefficients were calculated between averaged annual solar radiation levels for the period from 1961 through 1990 and cancer mortality in the year 2000 in 47 prefectures in Japan, with adjustments for regional per capita income and dietary factors. A moderate, inverse correlation with solar radiation was observed for cancers of the esophagus, stomach, colon, rectum, pancreas, and gallbladder and bile ducts in both sexes (correlation coefficient, ranging from -0.6 to -0.3). The results of this study support the hypothesis that increased exposure to solar radiation reduces the risk of cancers of the digestive organs.