OBJECTIVE Carcinosarcoma of the bladder is a rare neoplasm characterized by an intimate admixture of carcinoma and malignant soft tissue neoplasm. The clinical usefulness of separating carcinosarcoma (carcinoma with sarcomatous component) from sarcomatoid carcinoma (carcinoma with spindle cell carcinomatous component) is uncertain, and it comprises the subject of this report. METHODS We reviewed the clinical and pathological records of 10 men and 5 women a mean of 66 years old with carcinosarcoma, and 21 men and 5 women a mean of 66.5 years old with sarcomatoid carcinoma of the bladder, as documented in the files of the Mayo Clinic between 1936 and 1995. RESULTS Of the 15 patients in the carcinosarcoma group 9 had urothelial carcinoma, small cell carcinoma, 3 had squamous cell carcinoma and 2 had more than 1 type. The sarcomatous component included chondrosarcoma in 3 cases, leiomyosarcoma in 3, malignant fibrous histiocytoma in 3, osteosarcoma in 2, fibrosarcoma in 1, rhabdomyosarcoma in 1 and more than 1 type in 2. All disease was high stage at presentation. Treatment included cystectomy in 11 patients with (4) and without (7) radiation therapy, and transurethral resection in 4 with (1) and without (3) radiation therapy. Mean followup available in 14 cases was 34 months (range 1 to 144). A total of 11 patients died of cancer at 1 to 48 months (mean 17.2) and 2 survived for 8 to 131 months. Of the 26 patients in the sarcomatoid carcinoma group 18 had urothelial carcinoma, 1 had squamous carcinoma, 2 had urothelial carcinoma combined with squamous cell carcinoma and 5 had spindle cells only with no recognizable epithelium. All but 1 case was high stage at diagnosis. Treatment included transurethral resection in 17 patients with (7) and without (10) radiation therapy, including 1 who also received chemotherapy, and only cystectomy in 5, including 2 who also underwent radiation therapy and 1 who also received chemotherapy. Mean followup available in 21 cases was 49 months (range 1 to 420). A total of 17 patients died of cancer at 1 to 73 months (mean 9.8), 1 was alive at 140 months and 3 died of unrelated causes. CONCLUSIONS Carcinosarcoma and sarcomatoid carcinoma of the bladder are highly aggressive malignancies with a similar outcome regardless of histological findings and treatment. Pathological stage is the best predictor of survival.