The role of pulmonary function as an independent predictor of lung cancer risk was studied in a community-based cohort of 6,317 Japanese-American men who were aged 45 to 68 at the time of examination. After a follow-up period of about 22 yr, 172 incident cases of lung cancer were identified. The percentage of the predicted FEV1 was inversely related to lung cancer (p value for trend = 0.01) after adjustment for age and cigarette smoking history. The subjects in the lowest quartile of pulmonary function (% predicted FEV1 less than 84.5) had a relative risk of 2.1 (95% confidence interval = 1.3 to 3.5) for lung cancer compared with subjects in the highest quartile (% predicted FEV1 = 103.5+). For the 84 cases with a squamous or small cell histologic type of lung cancer, the subjects in the lowest quartile had a relative risk of 2.5 (95% Cl = 1.2 to 5.6) compared with subjects in the highest quartile of pulmonary function. For the 84 patients with lung cancer whose tumors were located within 4 cm of the pulmonary hilum, the subjects in the lowest quartile had a relative risk of 4.0 (95% Cl = 1.7 to 9.7). The results suggest that impaired pulmonary function in a community-based population is a predictor of lung cancer.