To determine the relationship between obesity, diabetes, and survival in a large cohort of breast cancer patients receiving modern chemotherapy and endocrine therapy.

We identified 6342 patients with stage I–III breast cancer treated between 1996 and 2005. Patients were evaluated according to body mass index (BMI) category and diabetes status.

In a multivariate model adjusted for body mass index, diabetes, medical comorbidities, patient- and tumor-related variables, and adjuvant therapies, relative to the normal weight, hazard ratios (HRs) for recurrence-free survival (RFS), overall survival (OS), and breast cancer-specific survival (BCSS) for the overweight were 1.18 [95% confidence interval (CI) 1.02–1.36], 1.20 (95% CI 1.00–1.42), and 1.21 (95% CI 0.98–1.48), respectively. HRs for RFS, OS, and BCSS for the obese were 1.13 (95% CI 0.98–1.31), 1.24 (95% CI 1.04–1.48), and 1.23 (95% CI 1.00–1.52), respectively. Subset analyses showed these differences were significant for the ER-positive, but not ER-negative or HER2-positive, groups. Relative to nondiabetics, HRs for diabetics for RFS, OS, and BCSS were 1.21 (95% CI 0.98–1.49), 1.39 (95% CI 1.10–1.77), and 1.04 (95% CI 0.75–1.45), respectively.

In patients receiving modern adjuvant therapies, obesity has a negative impact on RFS, OS, and BCSS; and diabetes has a negative impact on RFS and OS. Control of both may be important to improving survival in obese and diabetic breast cancer patients.