A population based case control study was conducted to determine if a single occupational factor, uranium mining, was associated with an increased risk of lung cancer in a predominantly nonsmoking population of Navajo indians. The cases in this study consisted of 32 Navajo males with primary lung cancer. Comparisons consisted of 64 Navajo males who had died of nonrespiratory cancer. Comparisons were matched to cases within 5 years of age and diagnosis date. Multiple sources were used for all subjects to obtain information concerning smoking and occupation, with particular reference to whether the subjects had ever been employed as uranium miners. A history of being employed as a uranium miner was documented for 23 of the 32 cases, and could not be documented for any of the comparisons. Smoking histories were available for 21 of the 23 affected cases but were unavailable for any of the comparisons. Eight of the 21 cases were nonsmokers. Of the 13 smokers, two smoked less than one cigarette per day, six smoked between one and three per day, and five smoked four to eight cigarettes daily. The statistical association between uranium mining and lung cancer was highly significant. The estimate of relative risk was infinite because no comparisons were exposed, but with use of the binomial distribution the lower 95 percent confidence limit for relative risk was 14.4. The median duration of employment in underground mines was 13 years, while the median duration from start of employment to diagnosis was 23.5 years. The median age of cases who developed lung cancer and were uranium miners was 44 years, while the median age for affected nonminers was 63 years. The authors conclude that uranium mining increases the risk of lung cancer in humans.