The three-dimensional structure of small intrahepatic bile ducts in congenital biliary atresia was examined by graphical reconstruction of serial histological sections. In the disease, intrahepatic bile duct system was constituted by an extremely complex network of proliferating bile ductules. Bile duct proliferation was interpreted as a reaction of the bile system to bile stagnation and bile leakage in the portal area. The restoration of a patent intrahepatic bile system seemed to depend upon the preservation of `main routes' of bile passage, which probably belonged to the pre-existing bile duct system. The maintenance of anatomically recognizable main routes could be correlated with the clinical effect of extrahepatic bile duct plasty.