Since animal experiments have shown that microcephaly is induced by fetal exposure to radiation and microcephaly has been found in children of mothers exposed to x-ray therapy during pregnancy (Murphy et al), the main cause of microcephaly in children exposed in utero to the A-bomb is considered to be ionizing radiation. Wood et al reported the increased incidence of microcephaly and mental retardation in children exposed in utero at proximal distances which they felt could not be attributed to any other known variable. ABCC has recently concluded that the effect of in utero exposure is primarily due to the immediate effect of radiation upon the fetuses although in A-bomb exposure the physical injury to the mother due to the A-bomb cannot be completely ignored. Our survey likewise revealed an increase of microcephaly in children exposed early in pregnancy at less than 15 weeks at closer distances than 1500 m. Thus, we presume that A-bomb radiation increases the incidence of microcephaly. 16 references, 8 tables.