A 32-year-old-woman with a history of endometriosis and chronic pelvic pain had left-sided pain and ultrasonographic documentation of a left pelvic complex cyst approximately 5 cm in diameter. Laparotomy revealed a left retroperitoneal cystic mass adjacent to the iliopsoas muscle and overlying the major pelvic vessels. The mass was dissected and excised. Histopathologic study revealed endometrioma and osseous metaplasia in a supernumerary ovary.