The relationship between occupational and leisure-time activity and the risk of breast cancer was analysed using data from a case-control study conducted in the Swiss Canton of Vaud between 1993 and 1998 on 246 incidents, histologically confirmed breast cancer cases and 374 controls below the age of 75 years, admitted to the same network of hospitals for acute, non neoplastic non hormone-related conditions. For occupational physical activity, the multivariate odds ratios (OR) for the highest versus the lowest level of physical activity were 0.6 (95% confidence interval, CI = 0.35-1.04) when aged 15 to 19 years, 0.5 (95% CI = 0.26-0.98) when aged 30 to 39 years, and 0.68 (95% CI = 0.36-1.28) when aged 50 to 59. For leisure time physical activity, the ORs were 0.4 (95% CI = 0.26-0.69), 0.5 (95% CI = 0.30-0.81), and 0.4 (95% CI = 0.22-0.80) for the highest versus the lowest level, respectively, in the three age groups, and an inverse trend in risk was significant in all groups. This study, based on one of the few European datasets on the issue, further suggests that physical activity is a favourable indicator of breast cancer risk.