Endometriosis of the urinary bladder is uncommon, and malignant transformation within vesical endometriosis is extremely rare. Vesical endometriosis and Mullerianosis can cause problems in differential diagnosis with vesical neoplasm, and, conversely, primary vesical neoplasm arising in endometriosis can be difficult to distinguish from secondary vesical involvement. Mullerianosis has rarely been described in the urinary bladder. A case of endometrioid adenocarcinoma of the urinary bladder is reported, which illustrates the difficulties in diagnosis and the importance of morphology and ancillary studies in establishing the correct diagnosis.