Acetaldehyde, the first product of ethanol oxidation, has been shown to stimulate collagen gene expression and to form protein-acetaldehyde adducts. Because little is known about these adducts in human liver tissue, we assessed, with an immunohistochemical procedure, the presence and location of acetaldehyde-protein adducts in liver biopsy specimens of alcoholic patients. In addition, we correlated the presence of adducts with the progression or subsequent occurrence of liver fibrosis. The group included 106 patients with high alcohol consumption (>90 gm ethanol/day for the last 5 yr), 10 nonalcoholic patients with normal livers and 23 patients with other liver diseases. Sixty-four of the 106 alcoholic patients had a second liver biopsy, whose specimen was used to assess the progression of liver fibrosis. Polyclonal antibodies were produced against homologous low-density lipo-protein purified from rabbit serum and modified in vitro in the presence of acetaldehyde. Protein-acetaldehyde adducts could be detected by immunohistochemistry in biopsy specimens of 90 alcoholic patients (85), in none of the 10 nonalcoholic patients with normal livers and in 65 of the patients with nonalcoholic liver disease. Acetaldehyde-modified epitopes were detected in the intracellular and extracellular compartment. Intracellular protein-acetaldehyde adducts were localized in the cytoplasm of hepatocytes with a more intense staining in zone 3. No correlation existed between the intensity of intracellular staining and the histologically assessed severity of liver disease. Extracellular acetaldehyde-modified epitopes were detected in 55 (52) biopsy specimens of alcoholic patients, with 33 of 39 (85) patients with alcoholic hepatitis or alcoholic hepatitis in cirrhosis, in none of the 10 nonalcoholic patients with normal livers and in 3 (13) of 23 patients with other liver diseases. Extracellular protein-acetaldehyde adducts were colocalized within the extracellular matrix. A brighter staining was seen in areas of histologically assessed active fibrogenesis and no or low staining in the well-organized older fibrous tissue. The presence of extracellular acetaldehyde adducts in the first biopsy specimen was significantly correlated to progression of liver fibrosis in the second biopsy specimen (p < 0.05). The results of our study indicate that covalent crosslinks between acetaldehyde and proteins could be involved in the pathogenesis of liver fibrosis. (Hepatology 1994;19:367–374).