Objective: to study the clinical significance of Pglycoprotein (P-gp) in breast cancer. Methods: Expression of P-gp in 60 cases of breast cancer was examined by immunohistochemistry. P-gp expression and response to chemotherapy were comparatively investigated in 19 patients with metastatic breast cancer. Results: The P-gp was positive in 48.3% of the 60 cases of breast cancer. P-gp expression was not related to patients age, menstruation status, number of axillary lymph nodes involved, clinical stage, histological type, and hormonal receptor status (P＞0.05). The frequency of metastasis (62.1%) and mortality (51.7%) were higher in P-gp positive cases than in negative cases (16.1% vs 12.9%, P＜0.005). The 5-year survival rate of P-gp positive cases (48.3%) was significantly lower than that of negative cases (87.1%) (P＜0.05). In patients who received adjuvant chemotherapy distant metastasis occurred more frequently in the P-gp positive cases (94.7%) than in the P-gp negative cases (57.1%) (P=0.0468). More P-gp negative patients (7/9) than positive patients (1/10) were responsive to chemotherapy (P=0.0055).Conclusion: Immunohistochemical examination of P-gp expression is useful in predicting response to chemotherapy and prognosis in breast cancer patients. P-gp positivity is associated with poor prognosis.