After the 1945 atomic bomb explosions, attention was initially drawn to the leukemogenic effect of atomic radiation. Mortality studies over the years have shown that the leukemogenic effect has been gradually declining, and the incidence of solid tumors has been increasing. There was shorter latent period for the appearance of excess leukemia mortality than for solid tumors. It appears that the risk for developing solid tumors becomes greatest when the population enters the age group at which that particular form of cancer most commonly occurs. The leukemogenic and carcinogenic risks are also greater in those exposed to radiation in the younger age groups. There are implications in these studies for cancer control with regard to radiation safety in diagnostic radiology and the peaceful uses of atomic energy; in addition, they emphasize the moral obligation involved in limiting the proliferation of atomic weapons. These observations may also be parallel to the experience of oncologists with second primary cancers after the administration of chemotherapy.