Background: Multiple reasons have been cited for racial disparities in breast cancer survival, including differences in screening, tumor biology, estrogen receptor status, stage at presentation, treatment and comorbidities. Recent studies suggest that minority women are more likely to have diabetes and diabetes has been shown to increase breast cancer risk and worsen cancer prognosis. We sought to evaluate whether there is disparity in breast cancer mortality among breast cancer survivors with comorbid diabetes. Methods: We identified women >65 years diagnosed with primary breast cancer between 2007 and 2010 from the SEER-Medicare database and followed them for 36 months after cancer diagnoses. Our primary outcome was breast cancer mortality. We excluded patients for whom diagnoses were obtained from death certificates or at autopsy, with incomplete data with regards to the stage at diagnosis and those without Part B coverage. The association between race and breast cancer mortality was assessed using a multivariable Cox regression with competing risk analysis. Results: A total of 9,545 breast cancer survivors with pre-existing diabetes were included in the analysis. Black women were more likely to be diagnosed at later stage and to be younger (p<0.001). There were no differences in cancer treatment by race. After a median follow-up of 36 months, 68.9% of black vs. 73.9% of white patients were still alive (p< .0001). After censoring women who died from other causes and adjusting for age, year of diagnosis, stage, cancer treatment (surgery, chemotherapy and/or radiation) and Charlson comorbidity score, black race remained associated with increased breast cancer mortality (Hazard Ratio 1.45 (95% CI, 1.20-1.73). Conclusions: Even after controlling for cancer-related treatment factors, black breast cancer survivors with diabetes have significantly worse breast cancer survival. Further research should investigate how to improve care for minority women who are at higher risk for breast cancer mortality.