Acetaldehyde, the primary metabolite of ethanol, binds covalently to proteins forming condensation products which have been recently shown to be immunogenic. To assess whether an antibody response against acetaldehyde-modified protein epitopes is associated with alcoholic liver disease, the serum immunoreactivity against proteins modified in vitro by acetaldehyde and against the corresponding unmodified proteins was measured by an enzyme-linked immunosorbent assay in 58 alcoholics with varying degrees of liver damage. Alcoholics showed significantly higher titers against protein-acetaldehyde conjugates than against the unmodified protein, independent of the nature of the carrier protein. The highest titers occurred in alcoholic hepatitis patients. Sera of patients with chronic hepatitis of nonalcoholic origin and of healthy controls also reacted with acetaldehyde conjugates, but their titers were significantly lower than those in alcoholic hepatitis patients. Our data support the idea that binding of acetaldehyde to proteins in humans generates antigenic determinants which trigger a corresponding immune response against such epitopes and suggest that this humoral immune response may be implicated in autoantibody formation and liver damage associated with excessive alcohol consumption.