The size and arrangement of each component of pancreaticobiliary ductal junction was investigated in 28 cases with biliary atresia in which cholangiography allowed visualization of the common bile duct, pancreatic duct, and duodenum and in which relevant data were available. As controls, seven cases of neonatal hepatitis and eight cases of intrahepatic bile duct hypoplasia were also studied. The following results were obtained. (1) The diameter of the common bile duct was shortest in biliary atresia, the value corresponding to about one-third of that for neonatal hepatitis. (2) The common bile duct was visualized in 100% of cases of intrahepatic bile duct hypoplasia and 78% of cases of biliary atresia, whereas the corresponding percentage was only 43% for neonatal hepatitis. The mean length of the common channel in cases of biliary atresia and that of intrahepatic bile duct hypoplasia was about 5 mm, which was longer than that of neonatal hepatitis. (3) The pancreatic duct type, in which the pancreatic duct merged into the bile duct, was frequent in cases of biliary atresia.