According to the 2008 survey conducted by Baba et al., the average prevalence of Japanese cedar-pollen allergy is 26.5% in Japan, although there are great regional differences in the prevalence of this disease. In this study, we investigated the causes of its regional differences.Regional differences in the 2008 prevalence of cedar-pollen allergy in 47 prefectures, obtained by Baba et al., were examined in terms of the correlations with the following factors: mean cedar pollen count in each region, the pollen dispersal period, relative humidity in February and March, and the concentrations of SPM, NOx and Ox. The pollen counts of cedar and cypress and the sum of these pollen counts were also measured.The prevalence of cedar-pollen allergy had the highest correlation with the pollen dispersal period, followed by the correlation with the pollen count and relative humidity in the order. There was no statistically significant correlation between the prevalence of cedar-pollen allergy and air pollutants such as SPM.