INTRODUCTION: Physical activity and body fatness affect breast cancer development and outcome. There is a tendency to reduce physical activity in the early stages of breast cancer treatment, but exercise interventions are rarely implemented during this period. OBJECTIVE: This study aimed to longitudinally assess the physical activity in patients with breast cancer during the first year after surgery and to investigate the factors associated with inactivity. DESIGN: A retrospective cohort study. SETTING: A university hospital. PARTICIPANTS: We reviewed the medical records of patients with breast cancer who visited the cancer rehabilitation clinic more than once. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Physical activity was assessed using the International Physical Activity Questionnaire. Body fatness was evaluated by calculating body mass index and body fat proportion obtained using bioimpedance. The physical function and health-related quality of life (HRQoL) were also assessed. RESULTS: A total of 130 patients were included. At initial evaluation, the total amount of physical activity was 1942.8 ± 2977.4 METs, with 46 (35.4%) participants categorized as inactive. The number of overweight/obese participants was 81 (62.3%), and 45 individuals had a high fat proportion (>
35.0%). At follow-up, there were no statistically significant changes in all parameters. However, the total amount of physical activity was increased to 2045.2 ± 2561.1 METs, and the number of inactive participants was reduced to 35 (26.2%) without statistical significance. Additionally, the number of overweight/obese participants was reduced to 76 (58.5%), and the number of participants with a high fat proportion was reduced to 35 (26.9%), without statistical significance. Physical function and HRQoL did not significantly change. The factors associated with inactivity included leg strength, chemotherapy at baseline, and body fat proportion at follow-up. CONCLUSIONS: Many patients with breast cancer remain inactive after surgery and some begin engaging in physical activity after a few months. Chemotherapy, poor strength, and a high fat proportion are considered barriers to physical activity.