Type 2 diabetes mellitus is associated with increased incidence and inferior outcome of various malignancies. The aim of this study was to explore the impact of type 2 diabetes on breast cancer characteristics at presentation. The study population included 79 diabetic and 158 age-matched non-diabetic patients. Parity, country of birth, co-morbidity other than diabetes, and mode of diagnosis were similar in both groups. Mean body mass index (BMI) was higher among diabetic patients. Tumour stage and size were higher among diabetic patients and the differences remained significant after adjustment for BMI. Moreover, after adjustment for BMI, breast cancer among diabetic patients was more often hormone receptor negative. Our results show that diabetes mellitus is associated with negative prognostic factors at breast cancer presentation.