Objective: To describe the anatomopathologic characteristics of endometriosis infiltrating the bladder detrusor. Design: Descriptive anatomopathologic study. Setting: Tertiary care center for endometriosis. Patient(s): Four patients, aged 22-38 years, who underwent laparotomy for bladder endometriosis. Intervention(s): Surgical excision and pathologic analysis of bladder endometriotic nodules in four patients. Main Outcome Measure(s): Gross and microscopic characteristics of endometriotic nodules. Result(s): A nodule of adenomyosis on the anterior wall of the uterus, in continuity with the detrusor lesion, was excised in three patients. In the other patient, bladder endometriosis was continuous with a nodule that infiltrated the left parametrium and extended as far as the posterior leaf of the ipsilateral broad ligament. Microscopically, all of the lesions had a similar histologic pattern: foci of endometriosis scattered in the bladder wall. The main feature was the paucity of endometrial-type stroma, particularly in the bladder submucosa, where glands were almost always dilated and cyst-like and were lined by flattened cells. Conclusion(s): Analysis of our patients, although it does not pinpoint the pathogenesis of the disease, seems to exclude the hypothesized intraperitoneal origin of endometriotic lesions of the bladder detrusor.