A 35-yr-old woman with a history of endometriosis and several recent episodes of hematuria was found to have an exophytic tumor arising from the mucosa of the posterior bladder wall. Microscopic examination showed a typical clear cell adenocarcinoma with a predominant tubulo-cystic pattern. A radical cystectomy was performed. Pathologic examination showed no residual tumor, but grossly visible intramural cysts at the site of the tumor had features of endometriosis on microscopic examination, strongly suggesting that the clear cell adenocarcinoma arose from endometriosis and was of müllerian derivation. Only one other adenocarcinoma of the bladder, an endometrioid adenocarcinoma, has had a probable müllerian origin. Although rare, these cases indicate that some vesical adenocarcinomas are of müllerian type.