A locus for progressive familial intrahepatic cholestasis (PFIC), also known as Byler disease, has been mapped to a 19 cM region of chromosome 18 by a search for shared segments, using patients from the Amish kindred in which the disorder was originally described. A similar liver disease, benign recurrent intrahepatic cholestasis (BRIC), recently has been mapped to the same region, suggesting that these two diseases are caused by mutations in the same gene. Although PFIC and BRIC are clinically distinct diseases, episodic attacks of jaundice and pruritus, with elevated concentrations of bile acid in serum, are seen in both disorders. In PFIC patients, these attacks result in progressive liver damage and death. The clinical and biochemical features of PFIC and BRIC are suggestive of a defect in primary bile acid secretion. The biology of bile secretion is of great interest because of its vital importance in digestion of dietary fats as well as in secretion of xenobiotics and metabolic waste products. Cloning of the gene (or genes) responsible for PFIC and BRIC will likely provide important insights into this pathway.