A 3-year-old healthy girl was seen in the emergency department after her father felt a mass in her abdomen while playing with her. She did not report any abdominal pain, nausea, vomiting, or weight loss, had normal bowel function, and denied any urinary symptoms. Physical examination revealed a slightly distended abdomen with a soft palpable mass measuring 10 to 12 cm in diameter that appeared to arise from the pelvis. Abdominal radiography showed a nonobstructive gas pattern. A computed tomographic scan of the abdomen and pelvis showed a 10 × 6 × 9-cm, solid and cystic heterogeneous mass occupying the right abdomen (Figure 1). Results of a complete laboratory workup including tumor markers were normal except for an elevated α-fetoprotein level of 14.7 ng/mL (reference range, 0-8 ng/mL; to convert to micrograms per liter, multiply by 1.0). Results of a metastatic workup including a computed tomographic scan of the chest and a bone marrow biopsy were negative. The patient underwent an elective exploratory laparotomy, which revealed a large intra-abdominal mass (Figure 2).