Background: The protease-activated receptor 1 (PAR1) is associated with increased invasiveness of breast cancer (BC) cell lines and its overexpression is significantly correlated with advanced stage and poor prognosis. The aim of this study was to determine the clinical significance of the serum PAR1 levels in BC patients. Methods: We enrolled 96 female patients with pathologically diagnosed BC who did not receive chemotherapy (CT) or radiotherapy. Serum PAR1 levels were measured by ELISA method and compared with 30 healthy controls.Results: The mean serum PAR1 level of BC patients was significantly higher than controls (3.07 vs 2.82 ng/ml, p = 0.011). The levels of PAR1 tended to be higher among CT responders (p = 0.052) and grade I disease than others (p = 0.055). However, there was no significant difference in PAR1 levels according to other clinic-pathological or laboratory parameters. Serum PAR1 level did not have a significant impact on overall survival in both univariate (p = 0.73) and multivariate analysis (p = 0.67).Conclusion: Serum PAR1 level is elevated in BC patients and may have predictive role for CT response. However, it has no prognostic role on survival.