The high incidence of anomalous pancreatobiliary duct union (APBDU) in children who have a choledochal cyst has been well documented. Additionally, cylindrical dilatation of the bile duct has been reproduced in animal models by anastomosing the pancreatic duct to the bile duct. In recent years, APBDU has been considered a possible etiologic factor in the formation of a choledochal cyst. The authors observed a progressive cylindrical dilatation of the common bile duct in a 6-year-old boy over an 18-month period. An operative cholangiogram showed a type Ic choledochal cyst and a type B APBDU (Todani classification). This clinical experience suggests that a normal common bile duct in children can be progressively dilated and become an acquired choledochal cyst arising as a complication of the preexisting APBDU.