Background: Numerous epidemiologic findings suggest that an increase in IGF-1, insulin concentration and body mass index correlates with the risk of developing certain types of malignant tumors, including breast cancer. They may also have an influence on the cancer stage. The purpose of this study was to prospectively evaluate the correlation of body mass index (BMI), insulin, and free IGF-1 concentration with cancer stage. Methods: Prospective analysis has been conducted in 2011 in Cancer Center and Institute of Oncology in Gliwice, Poland. This study has been approved by Bioethics Committee according to national regulations. The pilot group included 138 patients with breast cancer treated with chemotherapy, 76% in adjuvant setting and 24% due to cancer dissemination. Results: Patients’ median weight was 70kg (range 52-106kg) and median BMI was 27 (range 18.8-40.8). Patients with HER2 overexpression had diabetes more frequently (8% vs. 0%), p=0.06. Patients with higher BMI more frequently had negative steroid receptor status (42% vs. 25%) p=0.06, and significantly more often were in postmenopausal period (81% vs. 52%), p=0.00029. Tumor stage was significantly more advanced in patients with lower physical activity, p=0.04. Tumor stage was insignificantly more advanced in patients with higher plasma insulin level, p=0.09. Multivariate analysis had shown that overweight patients with more advanced stage of breast cancer had negative steroid receptors more often, p=0.033. Patients with metastatic breast cancer often had higher insulin plasma levels in comparison to patients with early breast cancer (7% vs. 1%), p=0.09. The multivariate analysis also showed that patients with diabetes had more frequently more often overexpression of HER2 and were in a more advanced cancer stage, p=0.03. Conclusions: Hyperinsulinemia may be a risk factor for more advanced stage of disease. There is a possibility that the effect of being overweight on breast cancer stage is mediated by estrogen metabolism. This analysis is scheduled to be performed later in the project.