Racial/ethnic disparities in mortality among US breast cancer patients are well documented. Our knowledge of the contribution of lifestyle factors to disease prognosis is based primarily on non-Latina Whites and is limited for Latina, African American, and Asian American women. To address this knowledge gap, the California Breast Cancer Survivorship Consortium (CBCSC) harmonized and pooled interview information (e.g., demographics, family history of breast cancer, parity, smoking, alcohol consumption) from six California-based breast cancer studies and assembled corresponding cancer registry data (clinical characteristics, mortality), resulting in 12,210 patients (6,501 non-Latina Whites, 2,060 African Americans, 2,032 Latinas, 1,505 Asian Americans, 112 other race/ethnicity) diagnosed with primary invasive breast cancer between 1993 and 2007. In total, 3,047 deaths (1,570 breast cancer specific) were observed with a mean (SD) follow-up of 8.3 (3.5) years. Cox proportional hazards regression models were fit to data to estimate hazards ratios (HRs) and 95 % confidence intervals (CIs) for overall and breast cancer-specific mortality. Compared with non-Latina Whites, the HR of breast cancer-specific mortality was 1.13 (95 % CI 0.97-1.33) for African Americans, 0.84 (95 % CI 0.70-1.00) for Latinas, and 0.60 (95 % CI 0.37-0.97) for Asian Americans after adjustment for age, tumor characteristics, and select lifestyle factors. The CBCSC represents a large and racially/ethnically diverse cohort of breast cancer patients from California. This cohort will enable analyses to jointly consider a variety of clinical, lifestyle, and contextual factors in attempting to explain the long-standing disparities in breast cancer outcomes.