Background: Protein Tyrosine Phosphatase Receptor-type O (PTPRO) has recently been in the spotlight as a tumor suppressor, whose encoding gene is frequently methylated in cancers.We examined the methylation status of the PTPRO gene promoter in breast cancer and evaluated the correlation between PTPRO promoter methylation and both clinicopathological parameters and prognosis of breast cancer patients.Methods: Two hundred twenty-one formalin-fixed, paraffin-embedded (FFPE) tumor tissues, 20 FFPE normal adjacent tissues and 24 matched plasma samples, collected from primary breast cancer patients, were assessed for PTPRO gene promoter methylation using methylation-specific PCR.Associations of promoter methylation with clinicopathological parameters were evaluated.Kaplan-Meier survival analysis and Cox proportional hazards models were used to estimate the effect on survival.Results: 175 samples gave identifiable PCR products, of which 130 cases (74.3%) had PTPRO gene promoter methylation.PTPRO methylation correlated with higher histological grade (P =0.028), but not other clinical parameters.Multivariate analysis indicated that overall survival (OS) was significantly poorer in HER2-positive, but not ER-positive patients with methylated-PTPRO.Methylated-PTPRO was detectable in matched plasma samples and only observed in plasma from patients whose corresponding primary tumors were also methylated.Conclusions: PTPRO methylation is a common event in the primary breast cancer and can be reliably detected in peripheral blood samples.PTPRO methylation is associated with poor survival only in HER2-positive patients, suggesting use of PTPRO methylation as a prognostic factor for breast cancer and for optimizing individualized therapy for HER2-positive patients.