Seventy five radiation-related leukemias (acute non-lymphocyte) in Hiroshima including 16 patients exposed to more than one Gray were cytogenetically examined. Statistical analysis of the data on the frequencies of chromosomal aberrations in survivors according to the bone marrow doses of DS86 estimation revealed that heavily exposed patients tended to have significantly higher aberration rates as compared with non-exposed patients. Furthermore, the chromosomal aberrations in the survivors were observed to be of a more complex nature and had characteristic findings of secondary leukemia. These observations therefore suggest that patients with a history of heavy exposure to atomic bomb radiation exhibit leukemic cells that originated from a stem cell which had been damaged by irradiation at the time of bombing and had been involved in the complex chromosome abnormalities. Molecular biological studies on transforming genes in acute and chronic leukemia and the bcr gene in chronic myelocytic leukemia have been performed in exposed and non-exposed groups. So far, no distinctive differences have been observed in the frequency and the sites of point mutations in N- and K-ras genes or in the rearrangement of the bcr gene, for a final conclusion of the specificity of radiation induced leukemia. Further retrospective studies require patient DNAs that developed in the early period of the atomic bomb exposure.