Choledochal cysts are congenital conditions associated with benign cystic dilatation of bile ducts. They are uncommon in Western countries but not as rare in East Asian nations like Japan and China.

They were classified into 5 types by Todani in 1977.

The classification was based on the site of the cyst or dilatation.

• type I: most common variety involving saccular or fusiform dilatation of a portion or entire common bile duct (CBD) with normal intrahepatic bile ducts

• type II: isolated diverticulum protruding from the CBD

• type III or choledochocele: arises from dilatation of the duodenal portion of CBD or where the pancreatic duct meets

• type IV: dilatation of both intrahepatic and extrahepatic biliary ducts

• type V or Caroli disease: cystic dilatation of intrahepatic biliary ducts