The Radiation Effects Research Foundation (RERF) continues to conduct a follow-up study initiated some years ago of cancer mortality and incidence among the survivors of the atomic bombing of Hiroshima and Nagasaki exposed in utero. Although only 18 incident cases of cancer were identified in the years 1950-1984 (of which 5 cases were in the 0 dose group), cancer risk appears to increase significantly as maternal uterine dose increases. Only two cases of childhood cancer were observed among these individuals in the first 14 years of life; both had been exposed to greater than or equal to 0.30 Gy. All other cases developed cancer in adulthood, and the cancers they developed are, in the main, the ones known to be elevated in frequency among the postnatally exposed survivors. The estimated relative risk for cancer at 1 Gy (uterine dose) is 3.77. The results suggest that the in utero group may have a higher risk than that seen among exposed adults because the individuals exposed in utero have not reached the major cancer prone age. However, since the observed cases are too few to allow a site-specific review, further follow-up studies are required to determine if the observed increased cancer risk can definitely be attributed to A-bomb radiation, although there appears to be a significant dose-related cancer response.