Transforming growth factor beta-1 (TGF-β1) participation in breast cancer development and metastasis is well-established, however, the clinical meaning of its circulating levels in women with breast cancer is poorly understood.To characterize the levels of TGF-β1 in plasma from women with breast cancer and to associate them with the main clinical factors associated with disease prognosis.TGF-β1 levels were measured by Enzyme-linked immunoassay (ELISA). Clinicopathological data were also assessed.Women bearing triple-negative tumors presented significantly reduced levels of this cytokine when compared to the other subtypes (p=0.0338). Patients with metastases exhibited lower levels of TGF-β1 than the non-metastatic cohort (p=0.0442). Patients with early-onset disease had the highest plasma TGF-β1 levels (p=0.0036). Doxorubicin chemotherapy induced a reduction in TGF-β1 level, promptly after drug infusion (p=0.0494). Patients with TGF-β1 levels lower than 20 pg/ml exhibited a tendency to have a reduced overall survival in a 40-month follow-up.Lower levels of circulating TGF-β1 are associated with a poor disease prognosis.