Background In several epidemiological studies, breast cancer risk has been reduced among women who reported high levels of occupational or leisure-time physical activity. We used data from a population-based case control study to evaluate the effect of occupational physical activity on breast cancer risk. Methods Two hundred-thirty three incident cases of breast cancer and 670 controls or their next of kin were interviewed for information on breast cancer risk factors and a complete job history. Physical activity level of jobs were classi®ed using a Department of Labor rating scheme. We calculated adjusted odds ratios for light and medium/heavy activity jobs compared to sedentary jobs. Results Odds ratios for women who held medium/heavy jobs for less than 10 years and more than ten years were, respectively, 0.7 (95% CI 0.4,1.3) and 1.7 (95% CI 0.9,3.3). Conclusions In these data there was no evidence that holding a job of medium/heavy activity reduced breast cancer risk. The study was limited by misclassi®cation inherent in the occupational exposure scheme and by the lack of information on leisure time physical activity. The modest risk increase for subjects holding a medium/heavy job for at least 10 years probably represents either confounding or random variation. Am.