Background: Because physical activity may affect hormonal concentrations and energy balance, we decided to investigate whether everyday exercise is related to the risk of breast cancer. Methods: During 1974-1978 and 1977-1983, a total of 25 624 women, 20-54 years of age at entry, enrolled in health surveys and answered questionnaires about leisure-time and work activity. Results: During a median follow-up of 13.7 years, we identified 351 cases of invasive breast cancer among the 25 624 women in the cohort. Greater leisure-time activity was associated with a reduced risk of breast cancer, after adjustments for age, body-mass index (the weight in kilograms divided by the square of the height in meters), height, parity and county of residence (relative risk, 0.63; 95% confidence interval, 0.42-0.95), among women who exercised regularly, as compared with sedentary women (P for trend = 0.04). In regularly exercising women, the reduction in risk was greater in premenopausal women than in postmenopausal women, and greater in younger women (< 45 years at study entry) than in older women ( 45 years) (relative risk, 0.38; 95% confidence interval, 0.19-0.79). In stratified analyses the risk of breast cancer was lowest in lean women (body-mass index, <22.8) who exercised at least 4 h per week (relative risk, 0.28; 95% confidence interval, 0.11-0.70). The risk was also reduced with higher levels of activity at work and again there was a more pronounced effect among premenopausal than postmenopausal women. Conclusions: Physical activity during leisure time and at work is associated with a reduced risk of breast cancer.