A fifty-seven-year-old woman with urinary bladder carcinoma with extensive areas resembling oat cell carcinoma of the lung in whom distant metastases developed, died seven months after diagnosis. Argyrophil cells could not be demonstrated, but electron microscopy demonstrated dense-core, membrane-bound intracytoplasmic granules. We reviewed 12 cases of epithelial neoplasms of the bladder from the literature in which there was ultrastructural evidence of neuroendocrine differentiation. Cases with malignant histologic features, like their pulmonary counterparts, have the potential for widespread dissemination and rapid growth. We support the previous suggestion that these neoplasms may be of considerable incidence and their recognition is important to determine prognosis and selection of therapy.