Current evidence suggests that PSC may affect the entire biliary system from interlobular bile ducts to the ampulla of Vater. If the affected bile ducts are too small for cholangiographic identification, the best designation for the condition is small-duct PSC; cholangiographically identifiable duct changes are the hallmark of large-duct PSC. The term "small-duct PSC" should replace the name pericholangitis because that designation lacks a clear definition. Small-duct PSC may occur in combination with large-duct PSC or it may occur alone. The clinical course of patients with PSC depends on the hepatic changes related to small-duct PSC, not primarily on the classic large-duct disease. Therefore the features of small-duct PSC and its complications are used to determine the disease stages of PSC. The incidence of small-duct PSC still is not known and the natural history as well as the pathogenesis of the condition remains obscure. Nevertheless, the role of small-duct disease in the syndrome of PSC has come into focus and the themes for future studies are apparent. Work should commence.