We report a case of degenerative extra-ovarian endometriosis. This is a rare complication of endometriosis, the frequency of which is estimated at 0.9%. In more than 50% of the cases, the diagnosis is made following investigation for abdominal pain. If the degeneration is of the same histological type as the endometriosis, it can be said to have arisen from the endometriosis. First-line treatment is surgery, removing as much endometriosis as possible, staging at this point is also necessary. Second-line treatment, with chemotherapy, radiotherapy and even hormonotherapy may be needed. Prognosis for this tumor is variable, from 10 to 100% five-year survival, depending on histological type and localization of the disease.