A 5-year-old boy with a 1-year history of intermittent abdominal pain presented to the emergency department with 2 days of nausea, nonbilious emesis, and diffuse abdominal pain. He was afebrile and had normal vital signs. Physical examination findings were notable for mild abdominal distension and tenderness without guarding or rebound sign. Abdominal ultrasonography revealed a normal appendix. Abdominal radiography ( Figure 1A) revealed multiple polygonal calcified lesions in the pelvis and a nonobstructive bowel gas pattern. Computed tomography ( Figure 1B) revealed a 5.6 × 5.3 × 4.6-cm midline cystic pelvic mass adjacent to the anterior rectal wall that contained several irregularly shaped ossifications. Laboratory study results were significant for normal levels of α-fetoprotein and chorionic gonadotropin and a white blood cell count of 19 700/μL (reference range, 5500-15 500/ μL; to convert to ×10 9 /L, multiply by 0.001). The patient was taken to the operating room for an exploratory laparotomy.