Plasma lipid peroxide levels were studied in 40 children with chronic cholestasis comprising 21 with syndromatic paucity of interlobular bile ducts (PILBD) and 19 with biliary atresia. Compared to the controls, mean lipid peroxide values were twice as high in children with biliary atresia (4.56 +/- 2.28 nmol/ml) and four times as high in those with PILBD (9.62 +/- 3.3 nmol/ml). These differences are highly significant. In patients with biliary atresia, the increase in lipid peroxide levels was clearly related to the bilirubin, cholesterol and phospholipid concentrations. In the PILBD group, however, there was little evidence of such a relationship. Vitamin E treatment seemed to have no effect on these increased lipid peroxide levels during the evolution of chronic cholestasis, and further investigations are necessary to clarify the pathological mechanisms involved.