OBJECTIVES: To discover how the age when a given dose of ionising radiation is received (exposure age) affects the subsequent cancer risk, and whether the types of cancer caused by repeated exposure to small doses during adult life differ from naturally occurring cancers at that age. METHOD: A nested case-control design with all possible controls in a cohort of nuclear workers, and a Mantel-Haenszel test (requiring only one degree of freedom) to discover whether there was any level of exposure age where the null hypothesis of no effects of radiation was rejected. This analysis was followed by inspection of how different types of cancers were related to the cancer risk. RESULTS: For radiation received at least 15 years before a cancer death (to allow for cancer latency) evidence of a dose related risk was found which was largely the result of exposures during the last 10 years of working life (between 55 and 65 years of age). The relative frequency of site specific cancers showed no signs of being different for radiogenic and idiopathic cancers, and there was no evidence of the exceptionally strong association between radiation and leukaemia found in atomic bomb data and other high dose situations. CONCLUSIONS: Sensitivity to carcinogenic effects of radiation increases progressively with age during adult life and, provided the dose is too small to produce many cell deaths, the ratio of leukaemias to solid tumours is no different for radiogenic and idiopathic cancers.