In 42 patients with locally advanced breast cancer treated with neoadjuvant chemotherapy followed by surgery and radiation therapy, the effects of chemotherapy on tumor architecture, morphometric nuclear and nucleolar characteristics, DNA ploidy, proliferation index measured by mitotic activity index, expression of differentiation antigens, and microvessel density were studied. Pretreatment biopsy specimens were available to compare with mastectomy specimens for 24 patients, and subclavicular biopsy specimens taken before chemotherapy were available for 9 patients. In the remaining patients, fine-needle aspiration was performed before chemotherapy, and morphologic and biologic features of the tumors could be studied only after chemotherapy. In 23 patients, only microscopic tumor or no tumor was left after chemotherapy, and in these patients we observed a characteristic pattern of relatively cellular fibrous tissue with lymphocytic infiltrate, ironloaded macrophages, and, when present, scattered foci of tumor cells in between. We found a reduction in mitotic activity index and in global microvessel density over all the tumors as a group. There was, however, no consistent pattern of changes in nuclear and nucleolar morphometric characteristics, DNA ploidy, and expression of differentiation antigens, and no pathologic or biologic features were predictive for response to chemotherapy.