A case with duplication of the duodenum found in the neonatal period is reported. A nine-day-old girl was referred with a history of frequent bilous vomitings since 7th day of life. The upper gastrointestinal X-ray, duodenal ultrasound and computed tomographic findings demonstrated severe narrowing of the second portion of the duodenum produced by an extraluminal cystic lesion. At operation, the duplication cyst was found to arise from the anti-mesenteric side of the second portion of the duodenum. The cyst was 3.5cm X 2cm X 2cm in size and projected against the true duodenal lumen, but no communication was present. The yellowish serous fluide was aspirated from the cyst. After the cyst was diagnosed as a duplication cyst, excision of the common wall between the duplication and the adjacent intestine was performed. Pathologic study showed that the musculature of the cyst was connected with the smooth musculature of the duodenal wall, The common wall was formed by the smooth musculature and the intestinal mucosal membrane. The patinet showed no other anomalies. Post operative course was uneventful and the patient was discharged at 37th day of life. Duplication cyst of the duodenum is a rare anomaly, representing only 10% of all duplications in the alimentary tract. According to the literature in Japan, 28 cases with duodenal duplication had been reported. Among these, only 6 cases were found in the neonatal period. The Bremer's theory is considered the best explanation of the development of the duplication in this case.
