A 10-YEAR-OLD GIRL was seen with a temperature of 102°F and a 4-month history of abdominal distension without pain, nausea, or vomiting. Findings from physical examination demonstrated a large mass filling her entire abdominal cavity. A computed tomographic scan showed a heterogeneous, large, abdominal mass measuring 20 × 11 × 20 cm (Figure 1). Her white blood cell count was 10.8 × 109/L with no left shift, and levels for blood urea nitrogen were 12.1 mmol/L; creatine, 92 µmol/L; α1-fetoprotein, 4.8 µg/L (reference range, <8.5 µg/L); and human chorionic gonadotropin assay, 3 IU/L (reference range, <5 IU/L). Antibiotics were administered for presumed urosepsis, and she underwent exploratory celiotomy. A large ovarian tumor was removed, measuring 29.0 × 19.5 × 10.5 cm and weighing 4.09 kg, and the capsule was intact. The cut surface was friable with multicystic areas (Figure 2). Findings from microscopic examination (Figure 3 and Figure 4) of the tumor showed solid areas, and tubules were lined by columnar to cuboidal cells with prominent mitotic activity. The patient was discharged, and no recurrence of the tumor was seen at follow-up 1 year later.