The histogenesis of phyllodes tumor (PT) and fibroadenoma (FA) is closely related, and discrimination between them by histopathological analysis is sometimes problematic. Moreover, objective criteria by which to categorize the grade of malignancy in PT are still controversial. The aim in this study is to clarify whether immunohistochemical evaluation using the MIB1 antibody, which reacts with the ki-67 antigen, correlates with the histological grade of malignancy in PT, and can discriminate between PT and FA. The 47 cases of phyllodes tumor (PT) were categorized into three groups (malignant, 4 cases; borderline, 6 cases; benign, 37 cases) according to the criteria proposed by Azzopardi (1979) and were investigated by immunohistochemistry. There were significant differences in stromal MIB1-index among the three groups (P < 0. 0001), and, unexpectedly, benign PT was easily divided into two groups according only to the MIB1-index. There were significant differences in the stromal MIB1-index (P < 0.001) and stromal cellularity (P < 0.01) between the two benign PT groups. A total of 478 cases of FA was reviewed and these were divided into 403 conventional fibroadenomas (CFA), 36 cellular fibroadenomas (CEFA) and 39 fibroadenomas with focal phyllodes structure (FAPS). All cases of CEFA and FAPS, and 140 cases of CFA were studied by immunohistochemistry. The 21/215 (9.8%) cases of FA, which were designated as FAMIB, showed a high stromal MlB1-index (more than 10/0.0625 mm2). Conversely, 77% cases of FA showed no MIB1-positive stromal cells. The incidence of MIB1-positive epithelium of FAMIB was much higher than that of FA. These results suggest that high proliferative activity may be present in both stromal and epithelial cells of FAMIB. Our study suggests that immunohistochemical evaluation using MIB1 antibody correlates with the histological grade of malignancy in PT, and can select FA with high proliferative activity. However, new objective diagnostic factors useful for discriminating FA from benign PT with a low MIB1-index should be developed.