Radiation-induced tumors cannot be distinguished from tumors in general by means other than a statistical excess. Epidemiological studies are the only means by which answers can be given as regards the carcinogenic effect of ionizing radiation. Age at exposure is perhaps the most important host factor influencing cancer risk and it is generally believed that cancer risk decreases with increasing age at exposure. For most cancers the temporal pattern follows the natural incidence, i.e. the cancers do not occur before ages normally associated with increased incidence. The induction period for solid tumors is at least 10 years while the corresponding figure for leukemia is 2 years. The breast, thyroid, lung and bone marrow seem to be the most radiosensitive tissues, while the risk of chronic lymphatic leukemia and possibly Hodgkin's disease and prostatic cancer does not seem to increase after exposure to ionizing radiation.