A 10-year-old boy presented with periumbilical postprandial pain and some melena. Physical examination was normal. All investigations were negative except a pertechnetate 99mTc abdominal scan which showed a very large and horn-shaped focus of high activity in the right flank. An ileal duplication was resected. It was lined by antral gastric mucosa with a large ulcer. The patient was treated successfully. The abdominal pertechnetate scan is discussed.