Cylindrical dilatation of the choledochus develops in 20% of patients with congenital bile duct dilatation and usually has acute-angled unions of the pancreatobiliary ductal system. Symptoms generally develop in patients over 1 year of age. The patients frequently complain of abdominal pain, vomiting, and fever as in those with acute pancreatitis. Ultrasonography and infusion cholangiography are the most useful tools in making a correct diagnosis. A high amylase level in the bile caused by the refluxing of pancreatic juice through anomalous ductal unions is commonly observed. This is responsible for biliary perforation in infancy and possibly carcinoma arising in the bile duct. The amylase concentration in the serum at the time of epigastric pain often is high, which leads to the diagnosis of acute pancreatitis. However, evidence of pancreatic inflammation is seldom noted. Accordingly, amylase in the bile may enter the circulating blood through the denuded epithelium or sinusoids of the liver. Excision of the whole extrahepatic duct along with hepaticoenterostomy would be essential for the treatment of cylindrical dilatation of the bile duct, especially when an anomalous ductal union is present.