BACKGROUND: The objective of this study was to assess the impact of pre-existing diabetes on breast cancer prognosis. METHODS: Women (n=2833) with centrally confirmed invasive breast cancer in the Women's Health Initiative, who were linked to Medicare claims data (CMS) were followed from the date of breast cancer diagnosis to date of death or 20 September 2013. Information on diabetes was identified through the CMS Chronic Condition Warehouse algorithm. Cox proportional hazard regression was used to estimate adjusted hazard ratios for overall mortality. A competing risks model (proportional subdistribution) model was used to estimate hazard ratios for breast cancer-specific mortality. RESULTS: Women with diabetes were more likely to have factors related to delayed diagnosis (less recent mammograms, and more advanced cancer stage) and were less likely to receive radiation therapy. Compared with women without diabetes, women with diabetes had significantly increased risk of overall mortality (HR=1.57, 95% CI: 1.23-2.01) and had nonsignificantly increased risk for breast cancer-specific mortality (HR=1.36, 95% CI: 0.86-2.15) before adjustment for factors related to delayed diagnosis and treatment. Adjustment for these factors resulted in a little change in the association of diabetes with overall mortality risk, but further attenuated the point estimate for breast cancer-specific mortality. CONCLUSIONS: Our study provides additional evidence that pre-existing diabetes increases the risk of total mortality among women with breast cancer. Very large studies with data on breast cancer risk factors, screening and diagnostic delays, treatment choices, and the biological influence of diabetes on breast cancer will be needed to determine whether diabetes also increases the risk for breast cancer-specific mortality.