Physical activity can improve quality of life (QOL) in breast cancer survivors but little is known about associations of physical activity and QOL during active cancer therapy. We examine associations between activity levels and QOL in a large cohort of breast cancer patients. Women with invasive, non-metastatic breast cancer (n=2,279) were enrolled between 2006 and 2009 from a managed care organization; assessment were done during active therapy. A physical activity frequency questionnaire was used to calculate the average weekly metabolic equivalent task (MET) hours spent in moderate and vigorous activity during active treatment. QOL was measured by the Functional Assessment of Cancer Therapy-Breast Cancer. Linear regression models tested cross-sectional associations of QOL and functional well-being with physical activity and covariates [socio-demographics, comorbidity, body mass index (BMI), clinical variables, social support, and assessment timing]. Physical activity had a significant positive unadjusted association with all QOL sub-scales (except emotional well-being) (all P values < 0.01). Overall QOL was 4.6 points higher for women in the highest quartile of moderate and vigorous activity versus women in the lowest quartile (P<0.001). In regression models, higher activity was associated with better overall QOL and functional well-being, controlling for covariates (P<0.05). Increasing BMI was also independently but inversely associated with overall QOL (P<0.001) but did not explain the relationship of activity and QOL. White women reported the higher levels of activity than minority women and activity was associated with QOL for Whites but not for minority women. Greater physical activity is associated with small but clinically meaningful increases in QOL during active breast cancer care therapy for Whites but this effect is not seen for minority women. If confirmed in longitudinal analyses, these differences may have implications for disparities research.