Background: Physical activity has been shown to reduce side-effects of breast cancer and its treatment. As physical activity levels of patients with breast cancer are largely unknown, we investigated these levels and compared them to women from the general population. Methods: In this prospective cohort study, physical activity levels of women with breast cancer participating in the UMBRELLA cohort were assessed at radiotherapy intake and thereafter every 6 months up to 3 years with the SQUASH questionnaire, which was also used in a random sample of the Dutch population. We compared physical activity levels (no activity, low, moderate or high levels of sports, leisure time or total activity) between patients and the Dutch female population using multinomial logistic regression. Standardized Prevalence Ratios (SPR) were calculated to compare adherence to Dutch physical activity guidelines. Results: Women with breast cancer ( n baseline = 1655, n 6 months = 1414, n 12 months = 1186, n 18 months = 957, n 24 ...