AbstractStudies of cancer among the Japanese survivors of the A-bombs dropped on Nagasaki and Hiroshima are the major source of information on radiation carcinogenesis in humans. They have already made important contributions to the estimation of the risk of radiation-induced cancer and to our understanding of key factors influencing risk, especially tissue sensitivity, age at exposure, and the temporal distribution of radiogenic cancers. The size of the exposed population still surviving virtually guarantees the continued productivity of the research conducted in Hiroshima and Nagasaki. It should not be supposed, however, that these studies will ever provide all the information needed for radiation protection standards or risk estimation. The experience of the A-bomb survivors simply does not include all important aspects of radiation exposure for which information is needed. Moreover, despite the size of the sample remaining under study, it is most unlikely that direct, empirical estimates can be made that will remove the necessity for dependence upon mathematical models to derive estimates of risk in the low-dose region.