Forty-seven cases of malignant mixed müllerian tumors were reviewed histologically and studied immunohistochemically with three major objectives: to analyze the histogenetic relationship between the carcinomatous and sarcomatous components of these neoplasms, to ascertain the practical role of immunohistochemical studies in diagnosis and classification, and to determine the prognostic significance of immunohistochemically verified rhabdomyoblastic and neuroendocrine differentiation. Epithelial differentiation (cytokeratin and/or epithelial membrane antigen expression) was confirmed in all carcinomatous components; within the sarcomatous areas, it was identified among individual cells (60% of cases) and within poorly formed clusters of cells (57% of cases). There was a statistically significant tendency for concordant expression of alpha-1-antichymotrypsin, Leu-M1, S-100, Leu-7, and neuron-specific enolase between the carcinomatous and sarcomatous components of individual cases. These two findings provide evidence of common origin for the sarcomatous and carcinomatous components of these neoplasms. Histologic review of metastases in 21 cases revealed a biphasic composition in the majority of metastatic lesions (62%), another feature that further supports a common origin for the two components. From a practical standpoint, immunohistochemistry may be helpful in accentuating the biphasic pattern of these neoplasms and in verifying the presence of rhabdomyoblastic differentiation. In most cases, however, careful morphologic examination and thorough sampling will suffice for correct diagnosis and subclassification. The presence of heterologous, rhabdomyoblastic, or neuroendocrine differentiation did not have a statistically significant influence on survival; the last of these was associated with a tendency for a more rapidly fatal course.