In a prospective study of 148 consecutive breast adenocarcinomas, proliferative indices of the same surgical tumor sample were performed by immunohistologic staining (Ki67 index) with the use of the Ki67 monoclonal antibody, which binds to a nuclear antigen only expressed in cycling cells, and by flow cytometry-derived S-phase fraction (SPF). Measurable Ki67 and SPF indices were obtained in 142 cases and 99 cases, respectively, and in 96 cases by both methods. In aneuploid tumors, a significant but low (P less than 0.05, r = 0.3) relationship was observed between Ki67 index and SPF. When compared with clinical, pathologic, and biochemical parameters these two proliferative indices were shown to be associated with nuclear grading and mitotic index. Additionally, correlations were observed between Ki67 index and node involvement (P less than 0.02) and between SPF and estrogen receptors (P = 0.002). These results show that (1) proliferative indices are obtained in 96% of surgical samples with Ki67 versus 67% with SPF and that (2) Ki67 index and SPF may provide complementary data with respect to prognosis.