The two largest studies of the effects of irradiation on humans in postnatal life are described. These are 1) the Ankylosing Spondylitis Study(ASS) carried out on patients given X-ray therapy as treatment for spondylitis in the UK and 2) the Life Span Study(LSS) carried out on the survivors of the atomic bombings of Hiroshima and Nagasaki. From these studies, a limited degree of subtyping of leukemias is indicated. Chronic lymphatic leukemia is much less readily induced by radiation than the other major subtypes. The inducibility of acute myeloid leukemia increases with age at exposure.