The monthly distribution of onset of leukemia during 1946-61 has been examined in 638 known cases among Hiroshima and Nagasaki atomic bomb survivors. Comparisons were made of sex, city, chronicity, and distance from the hypocenter. A summer excess was most prominent in the group 0-1999 m from the hypocenter, and an autumn deficiency was most consistently seen in the various subgroups. No explanation of these differences can be offered. 1 reference, 2 figures, 3 tables.