The clinical and pathological features of 15 previously unreported and 98 reported cases of nephrogenic adenoma were reviewed and compared with those of the 19 reported clear cell adenocarcinomas of the urinary bladder and urethra. Nephrogenic adenoma has occurred in patients from 4 to 83 (average 41) years of age. Two thirds of the patients were male, and most had a history of a genitourinary operation or procedure, genitourinary trauma, urinary calculi, or a renal transplant. Nephrogenic adenomas are typically small but have measured up to 7 cm. They may be papillary, polypoid, or sessile, and approximately 18% have been multiple. Microscopic examination shows, singly or in combination, tubules, cysts, and papillae lined by cells that typically have scanty cytoplasm and exhibit little cytological atypia and no or only very rare mitotic figures. Clear cell adenocarcinomas, in contrast, occur in a generally older age group, have a strong predilection for females, and typically lack the clinical features associated with nephrogenic adenomas. The carcinomas are almost always solitary and usually large. The microscopic findings of a diffuse growth pattern, clear cells with a high cytoplasmic glycogen content, and significant nuclear atypia and mitotic activity are characteristic of carcinomas in contrast to nephrogenic adenomas.