A 4-year-old boy presented with acute abdominal pain and a 1-year history of intermittent umbilical complaints. Ultrasonographic examination demonstrated a large cyst in the lower abdomen divided by a thin membrane. With micturition, impaired bladder function was expected. Initial suprapubic drainage produced 1,000 ml dark-brown liquid. Laparotomy revealed an ileal duplication cyst. Preoperative computed tomography and laparoscopy with the cyst drained did not further clarify the diagnosis.