Glutathione S-transferase (GST) enzymes detoxify carcinogens in tobacco smoke. Interindividual variation in GST function may be related to differences in risk for smoking-related cancer. Leukocytes from 50% of Caucasians lack GST activity toward trans-stilbene oxide (TSO), due to a deletion of the gene for the GST-mu enzyme. Presence of GST-TSO activity in leukocytes has been associated with low risk for lung cancer among cigarette smokers. We sought to determine whether GST activity in lung tissue is determined by the same gene polymorphism and whether it is associated with risk for lung cancer. Subjects were cigarette smokers, identified at the time of lung resection or autopsy in Seattle hospitals. Uninvolved lung tissue was obtained from 35 patients with lung carcinoma and 43 control patients and assayed for GST-mu activity with TSO, for the presence of the GST-mu gene product with an immunological assay, and for the GST-mu gene with Southern blotting. Mailed questionnaires were used to collect information on subjects' smoking histories and exposures which might alter enzyme activity. Interindividual results from the three assays correlated well. Smokers with high GST-TSO enzyme activity present in their lung tissue had a lower risk for lung carcinoma than did smokers with no or low activity (relative risk = 0.30; 95% confidence interval, 0.11-0.79), as did smokers with GST-mu antigen identified in lung tissue versus those with no antigen (relative risk = 0.30; 95% confidence interval, 0.11-0.79). Smokers with both maternal and paternal copies of GST-mu DNA (n = 7) had a lower cancer risk than smokers lacking GST-mu DNA (n = 30; relative risk = 0.35; 95% confidence interval, 0.06-2.10). High GST-mu activity appeared to be associated with a greater decrease in lung cancer risk among 38 heavy cigarette smokers (relative risk = 0.15; 95% confidence interval, 0.03-0.64) than among 38 light smokers (relative risk = 0.61; 95% confidence interval, 0.14-2.60). Presence or absence and number of copies of the GST-mu gene appear to determine activity of the GST-mu enzyme in lung. Smokers with the GST-mu enzyme have approximately one-third of the risk for lung carcinoma of smokers without the enzyme.