In a retrospective study, we investigated lung-cancer mortality from 1951 to 1976 in 1415 Swedish iron miners exposed to short-lived radioactive daughters of radon gas at concentrations leading to annual doses close to the currently accepted occupational limit. Fifty deaths from lung cancer were observed, as compared with 12.8 expected; expected rates were determined by a smoking-specific analysis based on data from a random sample of the Swedish male population. Among nonsmokers 18 deaths were observed, as compared with 1.8 expected; among current smokers and recent exsmokers 32 deaths were observed and 11.0 were expected. The effects of smoking and exposure to alpha radiation from radon daughters were nearly additive. Comparison of lung-cancer risk coefficients from this study and from other cohort studies of underground miners showed good agreement. Exposure to radon daughters is a major medical problem is underground metal mining, but our results also indicate that exposure to radon daughters at home accounts for an appreciable number of cases of lung cancer in the general population.