Ten cases of congenital extrahepatic biliary atresia were studied ultrastructurally. Samples of liver were obtained from each and in six of the cases, fibrous tracts, which we hoped would contain extrahepatic bile ducts, were also secured. The observed extrahepatic biliary structures were real, but hypoplastic, bile ducts. In places, necrosed epithelial cells, without obvious inflammatory processes, could be observed. The ductular cell cytoplasmic changes and the inflammatory reaction are different according to whether extrahepatic or intrahepatic sites are considered. These differences, as well as the cytoplasmic modifications of liver parenchymal cells, seem to be the result of impaired bile flow. On the contrary, extrahepatic bile duct hypoplasia and necrosis seem to be directly related to the unknown origin of this disease. Whether the nuclear changes of hepatocytes are the expression of direct injury of the liver is another important question. If there is direct injury, it is possible that the disease might evolve as an independent liver disease despite a correctly performed and uncomplicated surgical intervention.