The spectrum of diseases causing neonatal cholestasis presents intriguing problems for future investigation. There are many causes, and the eventual outcome of the specific entity has unique individual features, despite the wide areas of overlap. For example, extrahepatic biliary atresia may be the result of the sporadic occurrence of a virus-induced, progressive obliteration of the extrahepatic bile ducts with some degree of intrahepatic bile duct injury. This same sequence of viral infection with persisting injury may account for sporadic (nonfamilial) cases of neonatal hepatitis, as suggested by the Landing hypothesis. Conversely, the familial forms of cholestasis, either neonatal hepatitis or instances of intrahepatic cholestasis, are most likely genetic diseases that represent specific defects in the hepatic excretory process or in the bile secretory apparatus. The persistent nature of these presumed enzymatic or structural defects may explain the less favorable prognosis. Elucidation of the nature of these inborn errors of liver function may allow a better understanding of biliary physiology, and improved therapy.