OBJECTIVE Oat cell carcinoma of the urinary bladder is extremely uncommon. To date, 87 cases have been reported in the literature. Histologically, these tumors are similar to oat cell carcinoma of the lung and in slightly more than half of the cases the tumor is associated with another carcinomatous component: urothelial, squamous carcinoma, adenocarcinoma or, more rarely, sarcomatoid carcinoma. A case of sarcomatoid small cell carcinoma is described herein. RESULTS We report a case of sarcomatoid small cell carcinoma of the urinary bladder in a 79-year-old female patient. The tumor was in the advanced stages and was unamenable to resection. The patient was treated with multi-drug systemic chemotherapy, but she died five months after diagnosis. Light microscopy disclosed a carcinomatous component similar to oat cell carcinoma of the lung that stained with epithelial and neuroendocrine markers. The sarcomatoid component was histologically nondescript and showed extensive immunoreactivity to mesenchymal and epithelial markers. CONCLUSIONS Sarcomatoid small cell carcinoma of the bladder is rare. To our knowledge, only five cases have been previously reported. It is aggressive, metastasizes early and has a poor diagnosis. Surgical resection with adjuvant multidrug systemic chemotherapy is currently the best available treatment.