It has been suggested that type 2 diabetes mellitus may affect breast cancer prognosis, possibly due to increased diabetes-related comorbidity, or direct effects of insulin resistance and/or hyperinsulinemia. The aim of this study was to determine the impact of diabetes on disease-free survival (DFS) following mastectomy for breast cancer patients. The cases included in this retrospective study were selected from breast cancer women who had undergone mastectomy and completed adjuvant chemotherapy from 1998 to 2010. Patients were classified into two groups: diabetic and non-diabetic. Patients' age, sex, menopausal status, body mass index (BMI), histopathological features, tumor size, lymph node involvement, hormone receptor and HER2-neu status, and treatment types were recorded. There were 483 breast cancer patients included in the study. Postmenopausal patients' rate (53.7% vs. 36.8%, P = 0.016) and mean BMI levels were statistically higher (32.2 vs. 27.9, P = 0.007) in diabetic patients. There was no statistical difference for histological subgroup, grade, ER and PR positivity, HER2-neu overexpression rate, and tumor size between the diabetic and non-diabetic group. Lymph node involvements were statistically higher in diabetic patients compared with non-diabetic patients (P = 0.013). Median disease-free survival is 81 months (95% CI, 61.6-100.4) in non-diabetic patients and 36 months (95% CI, 13.6-58.4) in diabetic patients (P < 0.001). The odds ratio of recurrence was significantly increased in those with HER2-neu overexpression and lymph node involvement and decreased with PR-positive tumors. Our results suggest that diabetes is an independent prognostic factor for breast cancer.