Biphasic sarcomatoid carcinomas (SC) of the breast are defined as tumors that are composed of overtly carcinomatous and sarcoma-like elements and were known in the past as "carcinosarcomas." On the other hand, monophasic SC show features that are similar to those of sarcomas, but epithelial differentiation is detected in the former of these lesions by immunohistochemical methods. Adequate tissue sampling often is required to demonstrate both components of biphasic SC; if properly performed, this procedure greatly reduces the diagnosis of true mammary "sarcoma." The carcinomatous element of biphasic tumors is characterized either by features of a squamous carcinoma or an invasive ductal adenocarcinoma. Squamous differentiation is more often associated with a spindle-cell, fibrosarcoma-like, or malignant fibrous histiocytoma (MFH)-like sarcomatoid component, whereas adenocarcinomas usually are admixed with sarcoma-like tissues having "heterologous" properties. The overtly epithelial elements of biphasic SC are always immunoreactive for keratin, but areas that simulate sarcoma express this protein more heterogeneously. Both components may show positivity for vimentin, although the sarcoma-like elements do so more consistently. Monophasic SC is composed solely of fusiform and pleomorphic cells that are keratin-positive despite their mimicry of sarcomatous differentiation. This tumor variant commonly assumes a myxoid, angiomatoid, or storiform growth pattern, without recognizable carcinomatous foci by light microscopy. Prognosis and involvement of regional lymph nodes in mammary SC cases is controversial, because different tumors have been grouped together in the past for the analysis of these variables. However, SC generally has a less favorable outcome than that of ordinary invasive carcinomas of the breast.(ABSTRACT TRUNCATED AT 250 WORDS)