The clinicopathologic, immunocytochemical, and ultrastructural features of 12 small cell undifferentiated carcinomas (SCUCs) of the urinary bladder are herein reported. The patients ranged in age from 50 to 82 years (median, 76 years). Ten patients were male and two were female. Gross hematuria was the most frequent complaint (67%). Five patients died of disease (median survival, 4 months), three patients are still living with unresectable disease after 6, 10, and 12 months, and four patients were only recently diagnosed. Grossly, most of the tumors were large polypoid masses. On light-microscopic examination, they resembled the intermediate and oat cell variants of pulmonary SCUC. Eight SCUC were associated with other forms of in situ or invasive carcinoma, including transitional cell carcinoma (seven cases), adenocarcinoma (three cases), squamous cell carcinoma (three cases), spindle cell carcinoma (one case), and atypical carcinoid tumor (one case). Immunocytochemically, 11 of the 12 tumors expressed one or more epithelial cell markers (epithelial membrane antigen, human milk fat globule protein-2, cytokeratin). Eleven of the 12 SCUCs were also positive for one or more neuroendocrine markers (neuron specific enolase, chromogranin, Leu-7, vasoactive intestinal polypeptide, serotonin). Electron microscopy performed on seven tumors demonstrated dense-core granules (150-250 nm) in all cases, with tonofilaments in four cases, dendrite-like processes in two cases, and intercellular lumina in one case. The clinicopathologic features of 18 previously reported SCUC of the urinary bladder are also reviewed. SCUC arising in this location is an aggressive neoplasm that often demonstrates multidirectional differentiation, including the frequent but not invariable expression of neuroendocrine features.