Lipid peroxidation is an autocatalytic mechanism leading to oxidative destruction of cellular membranes. The deleterious consequences of this mechanism are related in part to the formation of reactive aldehydic products that bind to intra- or extracellular molecules to form adducts. Specific antibodies directed against malondialdehyde (MDA) and 4-hydroxynonenal (HNE) adducts, major aldehydic metabolites of lipid peroxidation, allowed us to investigate in situ, with an immunohistochemical procedure, the occurrence of lipid peroxidation in a panel of different chronic liver diseases. Intracellular HNE and MDA adducts were detected respectively in 24 of 39 cases (62%) and in 12 of 34 cases investigated (35%). They were localized mainly in the cytoplasm of hepatocytes, with the strongest staining observed in hemochromatosis, Wilson's disease, and in areas of acute alcoholic hepatitis in cases of alcoholic liver diseases. A peculiar pattern of immunostaining was observed in primary biliary cirrhosis where biliary cells of destroyed but also intact bile ducts strongly expressed HNE adducts. The liver extracellular matrix also displayed MDA adducts (30 of 34 cases, 88%) and HNE adducts (23 of 39 cases, 59%). While HNE adducts were specifically localized on large bundles of collagen fibers, MDA adducts were detected in a thin reticular network and in sinusoidal cells around portal tracts or fibrous septa. In conclusion, lipid peroxidation by-products are detectable in chronic liver diseases. Immunohistochemical results suggest that this mechanism is implicated very early in the pathogenesis of some of these diseases.