Kato, H. (Dept. of Statistics, Atomic Bomb Casualty Commission, 5–2 Hijiyama Park, Hiroshima, Japan 730). Mortality in children exposed to the A-bombs while in utero, 1945-1969. Amer J Epidem 93: 435-442, 1971.—Mortality has been observed for 24 years from birth to 1969 in a cohort of 1,300 persons exposed to the A-bombs while in utero in Hiroshima and Nagasaki. Mortality for the whole period of 24 years increased with radiation dose. This increase of mortality with dose occurred during the first year of life. No increase in mortality was observed during the next 9 years of life, but mortality again increased with dose after 10 years of age. No particular cause of death was found to be especially related to radiation dose except for perinatal deaths. Only three deaths from malignant neoplasms, including one leukemia, occurred in the whole sample. The increase in mortality with dose was observed only for those exposed to the A-bombs in the third trimester. These increases in mortality with dose are not attributable to other concomitant variables such as parental age, birth order, socioeconomic factors and so on.