606 Y.C. Jo, K.R. Joo, D.H. Kim, et al. A B Fig. 3. Computerized tomographic finding. Dilated intrahepatic duct (A) and a common bile duct and markedly distended the gallbladder (B) is shown. Fig. 4. Gross finding of the surgical specimen. Segments of the duodenum, duplication cyst, gallbladder and some portions of the pancreas are shown. Resection along the left margin of the cyst (arrow) (A) reveals the inner side of the cystic wall (B). A Kelly is protruding into the duodenum (B). The three criteria for the diagnosis of a duplication cyst are the presence of an intimate attachment to the gastrointesti- nal tract, a smooth muscle coat, and an alimentary mucosal A B Denuded mucosa layer Muscle layer Submucosa Mucosa Fig. 5. Microscopic finding of the cyst wall obtained from the sep- tum of the duplicated segment. Mucosal denudation and replace- ment by granulation tissue are seen. The cyst shares a common muscular layer with the duodenum (bottom) (H&E stain, ×100). lining (2). Of above three criteria, the presence of a smooth muscle coat is absolutely essential for the diagnosis of a dupli- cation cyst. In our case, although the lining epithelial cells were entirely denuded and replaced by inflamed granulation tissue due to increased intraluminal pressure and inflamma- tion, most of the cyst contained smooth muscle coats with foci of a neural plexus, which supported the diagnosis of a duplication cyst. The differential diagnosis of DDCs and choledochoceles demands particular attention. The principal distinguishing features between these two conditions are their histological characteristics. DDCs are covered both inside and outside by duodenal mucosa containing a distinct layer of smooth