Subjects who were exposed in utero to the atomic bombs in Hiroshima and Nagasaki, along with suitable controls, are examined annually at ABCC. Of the 1613 subjects in the study sample, 30 have gross mental retardation. Within 1500 m from the hypocenter the prevalence of mental retardation is 5 times as high as for the more distal subjects, and 6 to 15 weeks gestation was the most sensitive period. Even when subjects with possible explanations for their retardation are excluded the pattern of differences remains the same. All but two of the retarded subjects had a smaller than average head and for those who were within 1500 m this effect is accentuated. 17 references, 2 figures, 8 tables.