Twelve carcinomas of the urinary bladder with a prominent component of spindle-shaped cells (sarcomatoid carcinomas) that arose in patients from 60 to 83 (average, 71.5 years) years of age are reported. The seven male and five female patients typically complained of hematuria. Seven tumors were sessile and five were polypoid. On microscopic examination, the malignant spindle cells merged with in situ transitional cell carcinoma or various forms of invasive carcinoma, including transitional cell carcinoma (11 cases), adenocarcinoma (2 cases), squamous cell carcinoma (2 cases), and small cell undifferentiated carcinoma (2 cases). Immunocytochemical stains were performed in 11 cases; the spindle cells stained for cytokeratin, epithelial membrane antigen, and vimentin. Limited follow-up in this series does not allow for conclusions concerning differences in behavior between sarcomatoid carcinoma and conventional transitional cell carcinoma of similar grade and stage. However, the typically deep invasion of the former tumors and their histologic features indicate that they are highly aggressive neoplasms.