A case of an extrauterine Müllerian adenosarcoma of the pelvic peritoneum is described. The patient, a 16-year-old girl, presented with abdominal distension and pain. At laparotomy, a tumor was found originating from the left pelvic side wall. Light microscopic examination revealed a Müllerian adenosarcoma. Postoperatively she was treated with polychemotherapy. Seventy months after initial presentation she is still free of tumor.