To assess the risk of lung cancer in patients with chronic obstructive pulmonary disease, we matched, on the basis of age, sex, occupation, and smoking history, 113 persons ("cases") who had a forced expiratory volume in 1 second (FEV1) of 70% or less of predicted normal with 113 control persons who had an FEV1 of 85% or more. All persons were observed from 1973-74 through 1984 for a diagnosis of lung cancer, death from lung cancer, and death from any cause. At entry, subjects had an age range of 45 to 59 years; men numbered 186 and women 40. Histologically proven lung cancer developed in 9 cases and in 2 controls, all men. The rate of development of lung cancer was significantly different in the two groups (p = 0.024): the 10-year cumulative percentage was 8.8% for cases and 2.0% for controls. Overall 10-year survival was estimated to be 74.0% for cases and 91.1% for controls (p less than 0.001).