We examined the association between physical activity and lung cancer in a prospective cohort of 27,087 male smokers, ages 50-69 years, enrolled in the Alpha-Tocopherol, Beta Carotene Cancer Prevention (ATBC) Study. After an average of 10 years of follow-up, 1,442 lung cancer cases were diagnosed. Cox proportional hazards models were used to estimate the relative risk (RR) and 95% confidence intervals (CI) of lung cancer associated with self-reported occupational and leisure-time activity, adjusted for age, supplement group, body mass index, cigarettes smoked daily, years of smoking, education, energy intake and vegetable intake. There were no associations between occupational, leisure-time or combined categories of physical activity with lung cancer risk; however, age appeared to modify the effect of leisure-time activity (p = 0.02). Within increasing quartiles of age, the RRs (CI) for men active in leisure time compared to sedentary men were 0.77 (0.54-1.09), 0.74 (0.57-0.95), 1.09 (0.89-1.33) and 1.03 (0.88-1.21). These data suggest that among smokers, neither occupational nor leisure-time activity is associated with lung cancer risk. There may, however, be some modest risk reduction associated with leisure activity among younger smokers. Published 2002 Wiley-Liss, Inc.