History A 23-year-old woman was found, on routine pelvic examination, to have a firm, tender, fixed adnexal mass, approximately 7 cm in diameter, extending into the superior pelvic cul-de-sac. She denied having genitourinary and gastrointestinal symptoms. No history of pelvic or urinary tract infection was present, nor was pregnancy a possibility. Laboratory studies were noncontributory. An excretory urogram was normal. A barium enema study demonstrated an increase in the retrorectal space but was otherwise normal. An ultrasound study of the pelvis (longitudinal section) demonstrated a solid mass (Fig 1, arrowheads) in the pelvis. Diagnosis Neurofibroma of the sacral root (S-2). A 6-cm retroperitoneal mass was discovered at laparotomy, slightly to the right of the midline and below the sacral promontory, corresponding to the solid mass observed on ultrasonography, located behind the uterus. A biopsy specimen identified this mass as a neurofibroma; it was demonstrated on postoperative computed tomographic (CT) scan