Cystic dilatation of the cystic duct: a new type of biliary cyst. A case of a patient with a congenital cyst that was localized solely to the cystic duct is presented. We propose including this anomaly in the biliary cyst classifications, which currently include choledochal and intrahepatic and extrahepatic biliary cysts. The diagnosis and management of this new type of cyst in addition to other types of biliary cysts are discussed. 