Carcinosarcomas of the prostate gland are exceedingly rare, and previous reports exist on only seven of these neoplasms. The authors studied two such tumors, which occurred in 63- and 69-year-old patients. One of them had osseous metastases develop, which were treated unsuccessfully by irradiation and diethylstilbestrol therapy. The other patients is free of disease 15 months after radical prostatectomy. Both tumors contained an intimate mixture of carcinoma and sarcoma; patient 1 displayed foci of chondrosarcoma, osteosarcoma, and leiomyosarcoma, whereas patient 2 exhibited areas of chondrosarcoma, osteosarcoma, rhabdomyosarcoma, and angiosarcoma. The phenotypic nature of these tissues was confirmed by immunohistochemical studies, showing reactivity for vimentin, S-100 protein, desmin, actin, myoglobin, or Ulex europaeus I agglutinin. Conversely, the sarcomatous components lacked prostate-specific antigen, epithelial membrane antigen, and cytokeratin, whereas carcinomatous elements expressed these three markers. The authors' data support the existence of true carcinosarcomas of the prostate, that is, malignant neoplasms with conjoint epithelial and mesenchymal differentiation. The question of whether prostatic carcinosarcoma is an entity that is totally distinct from sarcomatoid or metaplastic carcinoma remains problematic.