BACKGROUND: Heterologous carcinosarcomas of the urinary bladder are rare neoplasms, the histogenesis of which is still disputed. METHODS: The clinicopathologic, immunohistochemical, and ultrastructural features of eight cases were analyzed. RESULTS: The patients, 5 males and 3 females, had a median age of 70 years. Gross hematuria, dysuria, and urinary tract infections were the main presenting symptoms. Tumors were solitary in all cases and located in the right wall in six cases. Tumor size ranged from 2 to 12 cm (median, 5 cm). Four patients died of disease 2, 6, 17, and 42 months postoperatively, respectively. Microscopically, the tumors consisted mostly of a varied mixture of high grade transitional cell carcinoma with chondrosarcoma, osteosarcoma, rhabdomyosarcoma, and undifferentiated spindle cell (leiomyosarcoma-like) components with occasional transitional features between one component and another. All tumors but one invaded the muscularis propria or the perivesical fatty tissue. Immunohistochemically, keratin expression was observed focally in the sarcoma component as well as the carcinoma component. Reactivity for vimentin, desmin, muscle specific actin, and S-100 protein was observed in poorly differentiated areas in addition to the expected positivity of each histologic subtype of sarcoma. Ultrastructurally, one tumor showed evidence of both epithelial and chondrosarcomatous differentiation, whereas three contained rhabdomyosarcomatous elements. CONCLUSIONS: On the basis of the current series and a review of 55 reports from the literature, primary heterologous carcinosarcoma of the urinary bladder proved to be a highly malignant type of neoplasm occurring predominantly in elderly males that was most often in an advanced stage at presentation and rapidly became lethal. Histogenetically, some heterologous carcinosarcomas should be regarded as a variant of sarcomatoid carcinoma (metaplastic carcinoma) that shows prominent heterologous differentiation