Liver biopsies and/or autopsy specimens from 17 children (ages 1 week to 5 years) with nonsyndromatic paucity of the interlobular bile ducts were studied by light and electron microscopy. Initial biopsies were obtained before 90 days of age from all patients, and two or more specimens were available from nine. No specific underlying condition was found in nine infants. The remaining cases were associated with Down's syndrome (n = 2), hypopituitarism (n = 2), cystic fibrosis (n = 1), alpha 1-antitrypsin deficiency (n = 1), cytomegalovirus (n = 1) and Ivemark syndrome (n = 1). Before 90 days of age, portal changes included duct paucity and fibrosis. Lobular changes were nonspecific, consisting of cholestasis, giant cell transformation, extramedullary hematopoiesis and perisinusoidal fibrosis. Duct paucity, portal fibrosis and perisinusoidal fibrosis persisted after 90 days. Cholestasis was mild or no longer apparent. Portal changes before 90 days of age appear to be sufficiently distinctive to microscopically distinguish nonsyndromatic from syndromatic paucity. Electron microscopic findings suggest that paucity in nonsyndromatic patients may result from a primary ductal insult: ultrastructural studies revealed bile duct destruction characterized by undulation and breaks in the basal lamina and infiltration of the epithelium by lymphocytes. Bile canalicular dilatation with blunting of microvilli and electron-dense material in the lumen, predominantly seen before 90 days, also reinforces the hypothesis of a primary ductal defect.