The aim of this historical (retrospective) cohort study was to investigate the relation between occupational exposure to radon daughters and subsequent mortality from lung cancer. Participants were former workers from the Radium Hill uranium mine, which operated in eastern South Australia from 1952 to 1961. A total of 2,574 workers were identified from mine records. Exposures to radon daughters were estimated from historical records of radon gas concentrations in the mine and from individual job histories. Exposures of underground workers were low by comparison with other mines of that period (mean 7.0 Working Level Months [WLM], median 3.0 WLM). Thirty-six percent of the cohort could not be traced beyond the end of employment at Radium Hill. Among those traced to the end of 1987, lung cancer mortality was increased relative to the Australian national population of the period (Standardized Mortality Ratio = 194, 95 percent confidence interval [CI] = 142-245). Compared with surface workers, lung cancer mortality was markedly increased in the underground workers with radon daughter exposures greater than 40 WLM (relative risk = 5.2, CI = 1.8-15.1). From the available information, we conclude that this increase is unlikely to be due to differences in smoking habits or other confounders. Taken together with the findings from other occupational studies, these results support current moves towards more stringent radiation control in the workplace, and underline the importance of research into the possible effects of domestic radon exposures.