Background: As breast cancer survivors live longer, non-cancer mortality competes with cancer mortality. However, many studies evaluating physical activity have not accounted for competing risks or adequately tested effect modification by menopausal status. Therefore, we aimed to assess the association between physical activity and mortality in women with breast cancer, accounting for menopausal status and competing risks. Methods: We analyzed 1000 breast cancer cases from the multicenter CAMA study in Mexico with 19-year follow-up. Usual hours of leisure-time moderate-to-vigorous physical activity before breast cancer diagnosis or symptoms were dichotomized at 2.5 h/week. Primary outcomes were overall, cancer, and non-cancer mortality. We used a two-stage analysis: (1) Cox model for overall mortality testing menopausal status as an effect modifier, and (2) Fine-Gray competing risks models in strata with significant effects. Models were adjusted for DAG-derived clinical, lifestyle, and socioeconomic confounders. Missing data were handled using Multiple Imputation by Chained Equations with Random Forest. Results: Menopausal status significantly modified the association between physical activity and overall mortality (p=0.035). In postmenopausal women (n=584), physical activity was associated with reduced mortality (Table), with no significant association in premenopausal women. Conclusions: In postmenopausal women with breast cancer, physical activity before diagnosis or symptoms was associated with lower overall mortality at long-term follow-up. This benefit reflects significantly lower cancer-specific mortality when accounting for competing non-cancer mortality. These findings underscore the importance of physical activity after menopause and highlight the need for methodological approaches that jointly consider menopausal status and competing risks in survivorship research. Association of physical activity and mortality outcomes in postmenopausal women with breast cancer Outcome HR/sHR (95% CI) p-value Overall mortality 0.59 (0.41-0.84) 0.004 Cancer mortality 0.63 (0.40-0.98) 0.042 Non-cancer mortality 0.63 (0.35-1.14) 0.125 Models adjusted for age, clinical stage, comorbidities, Diet Inflammatory Index, alcohol, smoking, education, residence, and healthcare access. HR = hazard ratio (Cox model); sHR = subdistribution hazard ratio (Fine-Gray models).