For children irradiated in utero and nonirradiated children of Hiroshima hematologic findings between the years 1950 to 57 have been compared, and no changes peculiar to the irradiated group were found. Despite several predisposing factors toward the development of iron deficiency in Japanese children, overt anemia was rare and distribution of hemoglobin levels was only slightly lower than reported for American and European children. Leukocyte levels, irrespective of age of the children, were found to be progressively falling in Hiroshima children so that by 1957 the values were distinctly lower than those reported for normal children in Japan and the United States. No cause for this change was apparent. 19 references, 7 figures, 3 tables.