The initial radiographs indicated a calcified mass in the abdomen. Both ultrasonography (Figure, B) and abdominal CT scan (Figure, C) were performed, which confirmed the presence of the mass. The differential diagnosis of an abdominal mass in an infant is broad and includes both benign and malignant etiologies. Benign lesions can include teratomas, omental or mesenteric cysts, fecalomas, and gastrointestinal duplications. Abdominal malignancies most commonly found in this age group are Wilms tumor, neuroblastoma, hepatoblastoma, and rhabdomyosarcoma.1 Because of calcifications in the mass noted on CT scan, teratoma was felt to be the most likely diagnosis. On closer review, distinct body parts, including a spinal column, skull, ribs, and tibia/fibula, were identified (Figure, D), leading to the diagnosis of fetus in fetu. The patient was discharged home and returned for resection 1 week later. Pathologic evaluation confirmed the diagnosis of mature teratoma consistent with fetus in fetu. The patient will have serial α-fetoprotein values measured every 3 months. Although the initial α-fetoprotein value was normal in this patient, malignant transformation of a recurrent teratoma remains a possibility and α-fetoprotein value is a useful screening test for this entity.