Tenascin-C (TNC) is a key molecule in tissue remodeling, and high levels are observed in many diseases, including heart failure, thrombosis, atherosclerosis, and cancer. High TNC expression by immunohistochemical analysis has been shown in invasive and metastasizing tissues from a variety of cancers, including colon, lung, brain, and breast. This study was conducted to investigate the serum level of TNC in breast cancer patients and its relationship with tumor progression and known prognostic parameters. Ninety-six breast cancer patients were enrolled into the study. Serum samples were obtained on first admission before adjuvant and metastatic treatments were given and at follow-up. Serum TNC levels were determined by the solid-phase sandwich enzyme-linked immunosorbent assay (ELISA) method. Median age of diagnosis was 48 years old (range, 29-80). Thirty-seven (39 %) patients had metastatic breast cancer. The mean TNC levels were found to be significantly higher in patients with breast cancer (344.1 ± 42.4 pg/mL) compared to those in healthy controls (137.2 ± 26.8 pg/mL) (p = 0.005). Serum TNC level in grade 3 tumors was found to be significantly higher than in grades 1-2 tumors (p = 0.04). No correlation was detected between serum TNC levels and other prognostic parameters analyzed, including presence of metastasis, lymph node involvement, and tumor size. Serum TNC level had no significantly adverse effect on survival in univariate and multivariate analyses (p = 0.65 and p = 0.85, respectively). In conclusion, although serum TNC levels are elevated, it has no predictive or prognostic roles on survival in breast cancer patients.