BACKGROUND: Regular physical activity is associated with improved symptom control in patients with breast cancer but its association with chemotherapy completion or response is unclear. METHODS: Using a prospective design, 1075 breast cancer patients receiving neoadjuvant chemotherapy between March 2012 and February 2017 were studied. Physical activity was assessed using the Global Physical Activity Questionnaire [GPAQ-16], quantified in standardised MET-h/wk. Chemotherapy completion was defined as the proportion of patients completing planned treatment course, requiring dose reduction, or requiring dose delay. Response was evaluated by pathologic complete response (pCR). Associations between physical activity and primary outcomes were assessed using multivariable logistic regression models. RESULTS: There was no differences between any chemotherapy completion outcome on the basis of physical activity classification. The percent of patients not completing planned treatment was 5.7% for ≦0.33 MET-h/wk, compared with 6.8% for 0.34-16.65 MET-h/wk, and 4.6% for ≥16.6 MET-h/wk (p = 0.52). No significant relationships were observed between physical activity dose classification and pCR for the overall cohort or upon stratification by clinical subtype. CONCLUSION: Future studies are required to further investigate the relationship between pre-treatment levels of physical activity and function on treatment completion and response in breast and other cancer populations. CLINICAL TRIAL REGISTRATION: NCT01993498.