Histopathological study of the remnant of extrahepatic bile ducts in 40 cases of so-called uncorrectable biliary atresia, upon which we operated the last three years, has been performed. The histological findings of the remnant were classified into three types. Only two cases were found to have type 1a ducts in the porta hepatis area, from which we can expect better prognosis postoperatively. We also found that as the patients become older, the size of the duct in the remnant becomes smaller and the hepatic fibrosis becomes more remarkable. Therefore the operation should be performed in the infant with this lesion as young as possible. As for the evaluation of operative results of hepatic portoenterostomy for this lesion, a proper evaluation can be made only in those cases in which a microscopic examination of the remnant of extrahepatic bile duct at the porta hepatis area has been adequately performed. Concerning the pathogenesis of biliary atresia, we presume that congenital abnormalities of bile ducts are a basic factor, and additional nonspecific inflammation and bile stasis complete its pathological condition.