A 6-month-old boy with rectal bleeding was admitted to the hospital. Two months before admission he had melena. On admission, a suspicious abdominal mass was palpated and the hemoglobin was 8.3 g/dl. Abdominal ultrasonography was not diagnostic. Abdominal scintigraphy with Tc-99m pertechnetate showed an abnormal tubular shaped area of increased radioactivity filling the entire abdomen and was highly suggestive of intestinal duplication. At operation, 55 cm of ileal duplication was resected. Histologic examination revealed that the duplication was lined by gastric mucosa. Preoperative abdominal Tc-99m pertechnetate scintigraphy was essential to making the diagnosis of ileal duplication.