In a retrospective study, liquid nitrogen preserved specimens from 50 women with primary breast cancer, who underwent surgery at the Beijing Institute for Cancer Research between June, 1986 and September, 1988, were investigated. All patients under this study were staged in TNM II or later, involved with axillary lymph node metastasis, and treated with systemic postoperative adjuvant chemotherapy. The median length of follow-up was 69 months. The expression of P-glycoprotein was investigated by means of immunohistochemistry, using a monoclonal antibody C219 specifically against P-glycoprotein and avidin-biotin peroxidase method. Positive staining for P-glycoprotein was found in 23 (46%) of the 50 patients. The P-glycoprotein expression negative group fared better than the group that was P-glycoprotein positive in overall survival curves (p = 0.0008, by the generalized Wilcoxon test). The prognostic effect of P-glycoprotein expression remained statistically significant (p = 0.0007) after adjustment by multivariate analysis (Cox's model) for other prognostic factors. It is demonstrated that P-glycoprotein expression is a significant and independent predictor of postoperative survival in breast cancer patients. The results of the present study suggest that P-glycoprotein expression might also influence the biological behavior of breast cancers.