Familial intrahepatic cholestasis is a confusing group of syndromes. Four forms are defined and discussed in detail ("arteriohepatic dysplasia," the Byler syndrome, the THCA syndrome, and Norwegian cholestasis). A comparison of the distinguishing characteristics of these syndromes demonstrates that they share many features, including areflexia, retinal degeneration, and paucity of the intrahepatic bile ducts on biopsy. Alternatively, some traits appear to be specific for a single syndrome: posterior embryotoxon and bony anomalies for arteriohepatic dysplasia, the presence of an abnormal bile acid for the THCA syndrome, and giant cell transformation for Norwegian cholestasis. These syndromes, although rare, merit complete evaluation because, as nature's experiments in bile formation, they represent models of cholestasis and may provide clues to the understanding both of other forms of cholestasis of unknown etiology and of the normal mechanisms of bile formation.