A 12-YEAR-OLD girl presented to the Pediatrics Clinic of Taipei Municipal Chung-Hsiao Hospital, Taipei, Taiwan, with several months' history of abdominal distension and mild dyspnea. Family and medical histories were noncontributory. Abdominal and pelvic examination showed a tender pelvic mass extending into the right costal margin. The other physical findings were insignificant. Ultrasonogram, abdominal x-ray films, and computed tomographic scan (Figure 1) showed a large, partially calcified intraperitoneal mass extending from the pelvis to the xiphoid process. Preoperative serum α-fetoprotein and human chorionic gonadotropin levels were within normal range. During a laparotomy, a large tumor of the right ovary with a capsular tear was found. The omentum and peritoneum were erythematous and finely granular. A right salpingo-oophorectomy and biopsy of the omental implant were performed. The right ovarian tumor measured 23×15×15 cm and weighed 1300 g. The capsule was ruptured and the tumor was solid and cystic. Microscopic sections are shown in Figure 2 and Figure 3. Postoperatively, the patient has been healthy with no recurrent disease for 31 months.