Transforming growth factor beta (TGF-beta)1 is thought to be involved in breast carcinogenesis. TGF-beta1 acts in an antiproliferative manner in the early stages of breast carcinogenesis, but promotes tumor progression and metastases in the advanced stages of the disease. No data have been published on serum TGF-beta1 in breast cancer. We investigated TGF-beta1 serum levels in patients with breast cancer (n=135), ductal carcinoma in situ (DCIS) I to III (n=67) or fibroadenoma (n=35), and in healthy women (n=40) to determine its value as a differentiation marker between malignant, pre-invasive and benign diseases and as a predictive marker for metastatic spread. Median (range) TGF-beta1 serum levels in patients with breast cancer, DCIS I-III or benign breast lesions and in healthy women were 48.8 (18-82.4) pg/mL, 45.3 (26.9-58.3) pg/mL, 47.2 (17.2-80.5) pg/mL and 51.6 (30.9-65.1) pg/mL, respectively (p=0.2). In breast cancer patients TGF-beta1 serum levels showed no statistically significant correlation with tumor stage, lymph node involvement, histological grade, estrogen receptor status and progesterone receptor status. Our data fail to indicate any correlation between serum TGF-beta1 levels and clinicopathological parameters of breast diseases. Serum TGF-beta1 levels do not provide clinical information in addition to established tumor markers.