Radiological studies in 60 case of congenital biliary dilatation showed that the stricture is the most significant etiologic factor for the condition of the bile duct. Cystic dilatation of extrahepatic bile duct was successfully created by the ligation of distalbile duct in infant rats. As for the etiologic factor of biliary dilation in our animal model which choledochopancreatic ductal anastomosis was performed in puppies, stricture nt the site of anastomosis was supposed to be more significant than the reflux of pancreatic juice. These clinical and experimental studies showed a definite evidence which the congenital stricture of the biliary tract associated with choledochopancreatic long common channel is the most important etiologic factor for the dilatation of the bile duct, particularly, in Type I (cystic dilatation), the stricture was supposed to be occured by the malfusion of the ventral pancreas and dorsal pancreas, at the site where the common bile duct was buried into the pancreatic parenchyma. In Type II (fusiform dilatation), the stricture occured at the site of intraduodenal common channel or choledochopancreatic ductul junction.