Jablon, S. (Dept. Statistics, Atomic Bomb Casualty Commission, U.S. Marine Corps Air Station, FPO, Seattle, Wash. 98764) and H. Kato. Sex ratio in offspring of survivors exposed prenatally to the atomic bombs in Hiroshima and Nagasaki. Amer J Epidem 93: 253–258, 1971.—Meyer, Merz and Diamond (Amer J Epidem 89: 619–635, 1969) have reported that females who had been exposed to radiation while in utero, in their own subsequent reproductive performance, had a significantly increased proportion of male offspring if the x-ray exposure occurred before the 30th week of fetal life. The present report by Jablon and Kato is based on a study of the reproductive performance through 1969 of 2,444 persons including some exposed while in utero to radiation from the Hiroshima or Nagasaki atomic bombs in 1945, and matched controls. For 1,272 females included in this group 345 births were recorded, and for the 1,172 males, 109 births. No relationship was found between the sex ratio in the offspring and the dose to the parent as a fetus, within any trimester of gestation at exposure, for either female or male parents.