As a preliminary epidemiological study, we evaluated the geographical correlation between estimated ambient ultraviolet B (UVB) levels and the mortality risk of leukemia in Japan. Ambient UVB levels were estimated from meteorological data for several successive periods. The standardized mortality ratio (SMR) was calculated for the 11 regions or 38 large cities and different times for nine types of leukemia [the International Classification of Diseases, ninth revision (ICD-9), ICD 200-208]. The ecological relationship was assessed by calculating Spearman's correlation coefficient. Among the nine types of leukemia, geographical correlation was found for two types of leukemia: "other malignant neoplasms of lymphoid and histiocytic tissue" (ICD 202) and "lymphoid leukemia" (ICD 204). The correlation coefficients between the SMR and UVB levels were statistically significant and ranged from 0.4 to 0.7 for the former and from 0.3 to 0.6 for the latter type of leukemia. This ecological study generated the hypothesis that UVB exposure may increase the risk of leukemias of lymphatic origin.