Physical activity is inversely related to mortality from respiratory diseases including lung cancer. Physical activity improves pulmonary function but its impact on lung-cancer risk has not been studied much. During 1972-1978, 53,242 men and 28,274 women, aged 20 to 49 years, participated in a population-based health survey and were followed until 31 December 1991. We observed a total of 413 men and 51 women with lung cancer. Leisure activity and work activity were assessed using a questionnaire in 4 categories. In a sub-cohort, physical activity was assessed twice at an interval of 3 to 5 years. Leisure but not work activity was inversely related to lung-cancer risk in men after adjustment for age, smoking habits, body-mass index and geographical residence (p for trend = 0.01). Men who exercised at least 4 hours a week had a lower risk than men who did not exercise [relative risk (RR) = 0.71; 95% confidence interval (CI) = 0.52-0.97]. Reduced risk of lung cancer was particularly marked for small-cell carcinoma (RR = 0.59; 95% CI = 0.38-0.94) and for adenocarcinoma (RR = 0.65; 95% CI = 0.41-1.05), with no association seen for squamous-cell carcinoma. In the sub-cohort in which physical activity was assessed twice, the risk of lung cancer was particularly reduced among men who were most active at both assessments (RR = 0.39; 95% CI = 0.18-0.85). No consistent association between physical activity and lung-cancer risk was observed among women. Our results suggest that leisure physical activity has a protective effect on lung-cancer risk in men. The small number of incident cases, combined with the narrow range of physical activity reported, may have limited our ability to detect an association between physical activity and lung cancer in women.