BACKGROUND: Assessment of tumor proliferative activity is considered to be the most powerful prognostic factor aside from axillary lymph node status. The purpose of this study is to assess the clinical value of measurement of proliferative activity using the MIB-1 labeling index in patients with breast cancer. METHODS: Surgical specimens from 36 patients with benign breast disorders and146 patients with breast cancer were investigated. The MIB-1 labeling index wasdetermined on the specimens stained by immunohistochemical methods as much as possible. Clinical factors associated with the MIB-1 labeling index were reviewed. RESULTS: The MIB-1 labeling index for non-proliferative disorders, proliferative disorders, and breast cancer was 3.4 +/-1.9%, 8.9 +/-6.2% and 20+/-12%, respectively. The MIB-1 labeling index and tumor size, lymph node metastasis status, and clinical stage according to the TNM classification correlated significantly. Survival rate was inversely correlated with the MIB-1 labeling index. No patientwith an MIB-1 labeling index of less than 10% had lymph node metastases, and all are alive without recurrence. Patients with an MIB-1 labeling index of over 30% had an extremely poor prognosis. CONCLUSION: The MIB-1 labeling index is very useful for predicting both either extremely good or extremely poor prognosis, and axillary lymph node metastasis