A cohort of 3469 males with at least 1 y of underground uranium mining experience in New Mexico was assembled and mortality followed up through 31 December 1985. The mean and median cumulative exposures for the cohort were 0.39 J h m-3 and 0.12 J h m-3 (111.4 and 35.0 Working Level Months [WLM]), respectively. Overall, mortality in the cohort was significantly increased (standardized mortality ratio [SMR] = 1.1, 95% confidence interval [CI] = 1.02-1.2) relative to the general population of the state. By cause, significant increases were observed for lung cancer (SMR = 4.0, 95% CI 3.1-5.1) and for external causes of death (SMR = 1.5, 95% CI 1.3-1.7). The risk of lung cancer increased for exposure categories above 100 WLM; the excess relative risk increased by 0.5% per mJ h m-3, 95% CI 0.2-1.5 (1.8% per WLM, 95% CI 0.7-5.4). Data were consistent with a multiplicative interaction between smoking and exposure to Rn progeny in an exponential relative risk model. The risk of lung cancer varied substantially with age at observation; the odds ratios rose more steeply with exposure to Rn progeny for those less than age 55 y at observation.