INTRODUCTION: The aim of this study was to investigate the presence and the expression levels of the interleukin 13 (I1-13) in the primary breast cancer tumour tissue in relation to the unchanged breast tissue in the same patients and to the breast tissue in the patients with benign breast disease, and to investigate the correlation between the IL-13 expression levels and the pathohistological factors, and between IL-13 expression and estrogens and progesterone receptor status. MATERIALS AND METHODS: 50 patients with invasive ductal breast cancer and 20 patients with benign breast diseases were included in this prospective case-control study. The three-step immunohistochemical staining was used for testing the levels of IL-13 expression and hormone receptor status. RESULTS: IL-13 was present in breast cancer tumour tissue, and in the surrounding unchanged tissue in the same patients, and in breast tissue in patients with benign breast disease. The expression of IL-13 was significantly higher in breast cancer tumour compared with surrounding tissue (P < 0.05) of the same, lymph node-positive patients. In addition, IL-13 expression was significantly higher in breast cancer tumour compared with breast tissue in patients with benign breast diseases (P < 0.01). There was significant correlation between IL-13 expression and tumour size in patients with lymph node-negative breast cancer (r = 0.405, P = 0.050). There was no significant correlation between IL-13 expression and the other pathohistological factors, and no significant correlation between IL-13 expression and the lymph node status. CONCLUSION: Obtained results suggest possible involvement of IL-13 in breast carcinogenesis.