BACKGROUNDS. Type 2 diabetes mellitus (DM) and breast cancer are common diseases in the developed world which share certain risk factors. There are several possible pathophysiological mechanisms which could explain increased incidence of breast cancer in patients with type 2 DM, including increased circulating insulin level. Because certain types of treatment for type 2 DM are associated with increased circulating insulin levels, the choice of treatment for type 2 DM could have an effect on the development and characteristics of breast cancer. The aim of our study was to establish whether pathohistological tumor characteristics and cancer stage in breast cancer patients with type 2 DM differ with respect to the different types of treatment received for type 2 DM. We hypothesised that the use of circulating insulin raising anti-diabetic drugs is associated with prognostically unfavorable pathohistological tumor characteristics and higher cancer stage in breast cancer patients. METHODS. 135 female breast cancer patients with type 2 DM were included in our study. The average age of patients was 66.6 years and their average body mass index was 30.4 kg/m2. All of the patients were surgically treated between the years 2005 and 2011 at the Department of Surgical Oncology at the Institute of Oncology Ljubljana. The data was collected on the type of treatment for type 2 DM, pathohistological tumor characteristics and cancer stage from the patients’ medical records. We assigned the patients to different groups depending on the type of treat ment for type 2 DM. We compared the mean patient age and body mass index between groups using one-way analysis of variance. Pathohistological tumor characteristics and cancer stage were compared using Fisher’s exact test. RESULTS. Breast cancer patients with type 2 DM treated with human insulin and insulin analogues had higher regional lymph nodes stage (p = 0.037) and borderline statistically significant higher tumor stage (p = 0.089) than patients treated only with diet or oral anti-diabetic drugs. There was no difference in pathohistological tumor characteristics or cancer stage between patients treated with circulating insulin raising anti-diabetic drugs and patients treated with diet or metformin only. CONCLUSIONS. Use of human insulin and insulin analogues is associated with higher regional lymph node and tumor stage in breast cancer patients with type 2 DM.