Physical inactivity may an important outcome in the prognosis of breast cancer. Physical activity levels decrease significantly for breast cancer patients following the treatment and remain low after oncology treatment is completed. The aim of this study was to determine physical activity levels and to examine associations among physical activity, comorbidity, functional capacity, peripheral muscle strength, and psychosocial status in breast cancer patients. Forty breast cancer survivors were included in this study. Demographic and clinical characteristics were recorded. Physical activity was assessed with a short form of the International Physical Activity Questionnaire (IPAQ). We defined comorbid diseases according to the Charlson Comorbidity Index (CCI). Functional capacity was evaluated with a six-minute walk test (6MWT). Peripheral muscle strength was measured for quadriceps femoris muscle with a hand-held dynamometer. Psychosocial status was measured with the Hospital Anxiety Depression Scale (HADS). Forty percent of patients were inactive. The IPAQ total score was significantly related with quadriceps muscle strength (r=0.492; p<0.001) and HADS depression score (r=0.341; p<0.05). Marked correlations were also observed between IPAQ walking score and quadriceps muscle strength (r=0.449; p<0.001), HADS depression score (r=0.341; p<0.05), and CCI (r=-0.433; p<0.001). The CCI score was also markedly associated with quadriceps muscle strength (r=-0.413; p<0.001). 6MWT distance was not significantly correlated with any of the parameters. Regression analyses revealed that psychosocial status and peripheral muscle strength were significant predictors of physical activity estimated with the IPAQ short form and when combined, they explained 35% of the variance. Comorbidities, peripheral muscle strength and psychosocial status partially explain the variability of physical activity level in breast cancer survivors. These results suggest that physical inactivity contributes to worsening health in breast cancer survivors.