Among patients with breast cancer, few studies have examined the pattern of change of physical activity levels over time or the predictive factors for this change. Particularly sparse are studies comparing pre-surgical physical activity levels with those 12 months post-surgery. Patients with a primary operable breast cancer (N = 267) filled in the Physical Activity Computerised Questionnaire before breast surgery and 1, 3, 6 and 12 months post-operatively. Patient-, disease- and treatment-related factors were prospectively collected. Total physical activity level and occupational, sport and household activity levels were significantly decreased the first month post-operatively and did not recover during the first year after surgery. 'Being employed' was a predictive factor for a larger decrease of the total activity level, comparing the pre-operative and 12 months post-surgery stages. Having a spouse, a pN2-3 lesion and over 20 lymph nodes dissected predicted a decrease in occupational activity. Advanced age and smoking behaviour predicted a decrease in sport activities, and not having a spouse predicted a decrease in household activities. This study showed that 1 year after breast cancer surgery, pre-operative physical activity levels were not recovered. Breast cancer patients, and in particular those at risk for a decreased physical activity level, should be identified, encouraged and guided to increase their activities.