Total cancer deaths were not increased among 2,074 women and 1,277 men who were fluoroscopically examined an average of 73 and 91 times, respectively, during lung-collapse therapy for tuberculosis (TB). Patients who did not receive this form of therapy (2,141 women and 1,418 men) and general population rates were used for comparison. All subjects were discharged alive from eight TB sanatoria in Massachusetts between 1930 and 1954; the average follow-up was 23 years. Deaths due to breast cancer were not increased among exposed females [standardized mortality ratio (SMR) = 1.0, n = 24], and SMRs greater than 2.1 could be excluded with 95% confidence. In contrast to other series, our inability to detect a breast cancer excess was likely due to lower breast doses (66 rad) and higher average ages at exposure (28 yr) and thus lower sensitivity. A deficit of lung cancer among exposed males and females was observed (SMR = 0.8, n = 26), even though increased risks have been observed among other populations exposed to similar dose levels. The estimated average lung dose was 91 rad, and SMRs greater than 1.2 for lung cancer could be excluded with 95% confidence. Overall, this study indicates that the radiation hazard of multiple low-dose exposures experienced over many years is not greater than currently accepted estimates for breast and lung cancer. For lung cancer the radiogenic risk may be less than predicted from high-dose, single-exposure studies.