A group of 286 adolescent children, all of whom were in utero at the time of the atomic bombing in Nagasaki, Japan, were examined as part of a long-term program to determine possible differences in growth and development that might be attributable to ionizing radiation. Three comparison groups were studied: group 1, children whose mothers were within 2,000 meters of the hypocenter (point on the ground above which the bomb exploded); group 2, whose mothers were located between 3,000 to 4,999 meters; and group 3, whose mothers were not in the city at the time of the bomb. The age at menarche and degree of epiphyseal closure in the wrist were determined. In addition, measurements were made of head circumference, standing and sitting heights, weight, and chest circumference. Differences were found which were consistent with a radiation effect. This was most significant in head circumference measurements.