Reduction of the number of interlobular bile ducts (BDP) has potentially significant physiologic consequences. Most of the disorders currently recognized as BDP involve an easily discernible reduction in duct numbers, but actual duct quantitation needs to be done routinely in the examination of liver biopsies in order to recognize and assay the importance of less marked reduction. Ducts are seldom if ever completely absent, and may not be uniformly reduced throughout the liver, so a wedge biopsy is the only completely satisfactory sample for the morphologic evaluation of duct numbers. In some conditions the number of ducts varies greatly with time. BDP may be only one aspect or manifestation of a disease primarily characterized by other features, or it may be the principal cause of distress and a major feature in defining a disorder. In a few conditions, the mechanism of the paucity has been quite well established as the destruction of previously existing ducts, but at present the basis for duct paucity in most conditions remain obscure. The treatment of BDP is similar in all conditions. Because of the wide range of circumstances in which BDP occurs, the prognosis is highly variable and depends on the underlying condition. In some patients BDP is associated with a long life of relatively good quality, in others it is associated with rapidly progressive liver disease, and in others with an intermediate course. In children particularly, conditions with BDP may be familial. In recent years there has been some advance in recognizing individual conditions associated with BDP, but many cases are still unclassifiable. Evaluation of a patient with BDP should be directed not only to diagnosing known conditions, but also to defining new ones, and should include family history, alpha 1-antitrypsin determinations, bile acid analyses, chromosome analysis, and consideration of virus infection. Additional efforts should be made to determine the pathogenesis of the duct paucity in individual conditions in order to prevent, arrest, or reverse the processes involved.