Abstract The expression of MIB-1, a marker of proliferation, has been measured in tumours from 61 patients with large (>4 cm) operable breast cancers (T 2,3 N 0,1M,) treated with primary systemic therapy; either with hormone therapy (27 patients), chemotherapy (17 patients) or hormone therapy followed by chemotherapy (17 patients), in order to determine its role in predicting response and survival. Staining was variable between tumours (1–89% of malignant cells staining). There was no correlation between MIB-1 expression and oestrogen receptor concentration when both parameters were measured as a continuous variable, but MIB-1 values were significantly lower in ER-rich tumours compared with tumours which were ER-poor. Response to hormone treatment was restricted to oestrogen receptor positive tumours. Tumours which responded to treatment also had significantly lower MIB-1 values than those which did not ( P< 0.004). In terms of response to chemotherapy, neither ER status or MIB-1 predicted for response. However, MIB-1 values were significantly higher in tumours proceeding to a complete pathological response as compared to those with a lesser degree of response ( P< 0.01). The overall survival of patients at 10 years was 50%. Neither ER status or MIB-1 predicted for survival following primary hormone therapy. However, MIB-1 predicted for survival following primary chemotherapy; the survival of patients with tumours which showed high MIB-1 expression was significantly better than the survival of those patients with tumours which showed low MIB-1 expression ( P< 0.003). In contrast, in the group of patients treated with chemotherapy after failed hormone therapy, survival was significantly worse in those with high MIB-1 expression ( P< 0.005).