Tissue histologically indistinguishable from endometrium was removed from the bladder of a 73-year-old man. The lesional tissue involved the right ureterovesical junction, producing hydronephrosis on the right side. The patient had an adenocarcinoma of the prostate and had been on estrogen therapy for 5 years before diagnosis of endometriosis. Two previous reports of endometriosis in male subjects, who were also on estrogen therapy, are reviewed briefly.