Under the cooperation between Atomic Bomb Disease Institute (ABDI) of Nagasaki University School of Medicine, Institute of Nuclear Medicine of Hiroshima University and Radiation Effect Research Foundation (RERF), the Life Span Study (LSS) on 93,741 survivors (fixed cohort) and the Open City Study (OCS) on all survivors (unfixed) irrespective of whether they belonged to LSS or not, have been conducted in parallel over 45 years to ensure reliable case detection. For diagnosis and subtyping of detected leukemias, we adopted the FAB classifcation for acute leukemias and for exposure dose of individual survivors, the new dosimetry system 1986 (DS86). In LSS, 231 leukemia cases were analysed. There was strong evidence of radiation-induced risks for acute myeloid leukemia (AML), acute lymphoid leukemia (ALL) and chronic myeloid leukemia (CML), but not for adult T-cell leukemia (an endemic disease in Nagasaki area) and chronic lymphocytic leukemia. There was also significant difference between three major types with respect to the effects of age at bombing and sex, and in the temporal pattern of the elevated risks. For AML the dose response function was non-linear, whereas there was no evidence against linearity for ALL and CML. The hypothesis of a 0.5 Gy threshold could be rejected for three major types of leukemia. Excess Absolute Risk (EAR) estimates in cases per 10,000 Person Year Sievert (PYSv) were 0.6, 1.1, 0.9 for ALL, AML and CML, respectively. The corresponding relative risk at 1.0 Sv were 9.1, 3.3, 6.2, respectively. Although childhood exposure <15 age at bombing apparently induced three major types, the agerelated highest risk was observed for ALL. In OCS, 413 cases with DS86 estimates were used for analysis. Type specific incidence rates were calculated indirectly by using the over all incidence of leukemia from LSS data and multiplying these values by the corresponding proportions of cases in OCS. In conjunction with LSS data, the effects of radiation were significantly greater on the incidences of ALL and CML than on that of AML. In the high dose group there was a strong evidence for shorter incubation time and faster decline of elevated risk for ALL and CML than for AML. AML risk was apparently persistent through 1980. Thus, the differential effects of atomic bomb irradiation in inducing three major types of leukemia with respect of age-related and temporal patterns provide us insights into human leukemogenesis. Further investigation on radiation leukememogenesis undoubtedly requires molecular approach to detect type-specific genetic abnormlities including oncogenes and anti-oncogenes
