Low-grade adenosquamous carcinoma of the breast is a variant of metaplastic mammary carcinoma characterized by a locally invasive growth pattern and a low risk for metastases. In this study none of the carcinomas exhibited greater than 5 percent nuclear immunoreactivity for estrogen or progesterone receptors, and as a result they were classified as negative for these receptors. Reactivity for cathepsin D was found in 39 percent of the tumors, largely limited to areas of epidermoid differentiation. Membrane immunoreactivity for HER-2/neu oncogenes was present in glandular components of 46 percent of the carcinomas. Immunoreactivity for p53 (greater than 10 percent of nuclei) was present in 13 percent of the tumors, also in glandular elements. Six different patterns of coexpression of p53, HER-2/neu and cathepsin D were found, the most frequent being the following: HER-2/neu(+), p53(-), cathepsin D(-) (9 cases, 39%); cathepsin D(+), p53(-), HER-2/neu(-) (5 cases, 22%); and the three markers negative (5 cases, 22 percent). Coexpression of the two oncogenes was found in only one tumor which was also positive for cathepsin D. These results indicate that the expression of various immunohistochemical prognostic markers may be heterogeneous and that there may not be a specific pattern of marker coexpression within a carefully defined histologic subtype of mammary carcinoma. Furthermore, characteristics reported to be associated with an unfavorable prognosis (negative hormone receptors, presence of cathepsin D, and expression of oncogenes such as HER-2/neu) may be found in a substantial proportion of tumors that comprise this clinically and histologically low-grade variant of mammary carcinoma. This disassociation between expected prognosis based on expression of current prognostic markers and observed prognosis occurs in other forms of mammary carcinoma. Medullary carcinoma, when diagnosed on the basis of rigorously defined criteria, has an excellent prognosis despite the fact that these tumors are characterized by absence of estrogen and progesterone receptors and a high proliferative rate. The histological classification of mammary carcinomas is itself an important prognostic variable that may take precedence over selected biochemical markers.