To clarify the pathogenetic role of acetaldehyde in the development of alcoholic liver disease, genotyping of aldehyde dehydrogenase-2 genes was performed and the clinical features of the alcoholic liver disease patients with different genotypes were compared. Genotyping of aldehyde dehydrogenase-2 was performed in 47 patients with alcoholic liver disease using the polymerase chain reaction and slot-blot hybridization. Of the 47 patients with alcoholic liver disease, 40 were homozygous for the normal aldehyde dehydrogenase-2 gene and the remaining seven cases were heterozygous for the normal and mutant aldehyde dehydrogenase-2 genes. No homozygote was found for the mutant aldehyde dehydrogenase-2 genes. Daily alcohol intake was less than 100 gm in all heterozygotes without relation to the type of alcoholic liver disease. On the other hand, all but four patients homozygotic for the normal aldehyde dehydrogenase-2 gene drank more than 100 gm alcohol/day. The mean daily alcohol intake in the heterozygotes was significantly lower than that in the normal homozygotes. The incidence of alcoholic fibrosis tended to be lower in the heterozygotes than in the normal homozygotes (14.2% vs. 52.5%). On the other hand, the incidence of alcoholic hepatitis and/or cirrhosis tended to be higher in the heterozygotes than in the normal homozygotes. These results indicate that alcoholic liver disease develops even with moderate amounts of alcohol intake in heterozygotes of the aldehyde dehydrogenase-2 genes, in which acetaldehyde metabolism in the liver is impaired and liver damage in the heterozygotes is more severe than that in the normal homozygotes, suggesting that habitual drinkers who are heterozygotes of the aldehyde dehydrogenase-2 genes may be at high risk for alcoholic liver disease.