Background and aim Breast cancer is the most common female cancer disease of which are affected one quarter of women treated for cancer in the world and Croatia. The aim of this study was to explore the relationship of the molecule TGF β rIII in samples of plasma of the patients with breast cancer and tumour tissue with prognosis of the disease. Subjects and Methods The study included 62 female patients with breast cancer and 36 respondents in the control group. ELISA method was used to determine the concentration of the TGF β rIII in the patients' plasma before and after the surgery, while immunohistochemical analysis was used for expression of TGF β rIII protein in breast tumor samples. Results In this research for the first time we measured the values of the TGF β rIII in plasma of the patients with breast cancer and in plasma of healthy respondents. The average concentration of the TGF β rIII in plasma of the patients with breast cancer was 7,63 ng/ml (range 14,41 ng/ml), which was significantly lower concentration compared to the control group (p<0,001). Patients with breast cancer with preoperative concentration of the TGF β rIII in plasma lower than 7,83 ng/ml had statistically significantly lower survival rate without adversely primary outcome (metastasis, local recurrence or appearance of new primary tumors) compared to other patients (p=0,045). Patients with poor prognostic parameters (LN+, ER/PR-, Her2+) and with preoperative concentration of the TGF β rIII in plasma higher than 7,83 ng/ml had statistically significantly better survival rate without adversely primary outcome compared to patients with lower concentration of the TGF β rIII in plasma (p=0,016). Conclusion This study demonstrated that TGF β rIII in the plasma of patients with breast cancer disease could be a new predictor of outcome in a group of patients with low-risk disease and is an interesting subject for future research.