Earlier reports of progressive decline in leukocyte counts in Hiroshima from about 1948 to 1954 have been confirmed. A similar phenomenon has been observed in Nagasaki. Analysis indicates that this decline in white cell count with time is not related to exposure to the 1945 atomic bombs, to sex, to age, to commonly diagnosed diseases, or to the disproportionate influence of a subgroup. The principal white cells affected were neutrophils, lymphocytes and eosinophils. The precise etiologic factors accounting for the decline, and the biological significance of the present lower range of leukocyte values in Hiroshima and Nagasaki remain to be determined. 16 references, 5 figures, 5 tables.