Racial/ethnic disparity in breast cancer-specific mortality in the United States is well documented. We examined whether accounting for racial/ethnic differences in the prevalence of clinical, patient, and lifestyle and contextual factors that are associated with breast cancer-specific mortality can explain this disparity. The California Breast Cancer Survivorship Consortium combined interview data from six California-based breast cancer studies with cancer registry data to create a large, racially diverse cohort of women with primary invasive breast cancer. We examined the contribution of variables in a previously reported Cox regression baseline model plus additional contextual, physical activity, body size, and comorbidity variables to the racial/ethnic disparity in breast cancer-specific mortality. The cohort comprised 12,098 women. Fifty-four percent were non-Latina Whites, 17% African Americans, 17% Latinas, and 12% Asian Americans. In a model adjusting only for age and study, breast cancer-specific HRs relative to Whites were 1.69 (95% CI, 1.46-1.96), 1.00 (0.84-1.19), and 0.52 (0.33-0.85) for African Americans, Latinas, and Asian Americans, respectively. Adjusting for baseline-model variables decreased disparity primarily by reducing the HR for African Americans to 1.13 (0.96-1.33). The most influential variables were related to disease characteristics, neighborhood socioeconomic status, and smoking status at diagnosis. Other variables had negligible impact on disparity. Although contextual, physical activity, body size, and comorbidity variables may influence breast cancer-specific mortality, they do not explain racial/ethnic mortality disparity. Other factors besides those investigated here may explain the existing racial/ethnic disparity in mortality. Cancer Epidemiol Biomarkers Prev; 25(7); 1064-72. ©2016 AACR.