The prognostic value of tumor cell proliferative activity as measured by the MIB-1 monoclonal antibody in invasive ductal not otherwise specified breast carcinomas was determined for 186 patients, including 111 with no axillary node involvement. The MIB-1 antibody detects the Ki-67 antigen in microwave-processed paraffin sections of the formalin-fixed tumors. The mean MIB-1 score was 16% for all tumors, 16% for the node-negative group, and 15% for the node-positive group. In univariate survival analysis, the MIB-1 score (dichotomized, </=10 versus >/=10%) predicted overall 5-year survival in all of these groups. The mean MIB-1 score was significantly higher in vimentin- and p53 protein-positive tumors (P > 0.001) than in negative ones. The impact of vimentin expression and of p53 positivity on the prognostic significance of the tumor cell proliferation rate was assessed. Vimentin was associated significantly with poor 5-year survival in the entire cohort, and a particularly strong association was found in the node-negative group. p53 had a weak but statistically nonsignificant influence on survival. In a multivariate analysis using the Cox proportional hazards model, vimentin (P = 0.0002) was the only independent prognostic factor in node-negative patients. In contrast, the MIB-1 score (P = 0.009) was the only independent prognostic factor in the node-positive group. Therefore, node-negative patients with vimentin-positive tumors and node-positive patients with tumors with high proliferation rates might be appropriate candidates for early adjuvant chemotherapy.