The hepatic activities and the isoenzyme patterns of both alcohol dehydrogenase and aldehyde dehydrogenase have been studied in 60 alcoholics with varying degrees of liver damage (from normal tissue or minimal changes to cirrhosis with alcoholic hepatitis) and in 24 nonalcoholics with chronic hepatitis or cirrhosis in order to ascertain their association with liver damage. In alcoholics both alcohol dehydrogenase and low-Km aldehyde dehydrogenase activities decreased proportionally with the severity of liver disease. In contrast, in nonalcoholics, there was a reduction of low-Km aldehyde dehydrogenase activity related to the severity of liver injury, but alcohol dehydrogenase was similar in patients with chronic hepatitis and nonalcoholic cirrhosis. There were no significant changes in total and high-Km aldehyde dehydrogenase activities among the different histologic groups studied, although the lowest activities were observed in patients with more severe liver injury. The prevalence of atypical alcohol dehydrogenase was similar in alcoholic (6.6%) and in nonalcoholic (8.3%) liver disease. All patients exhibited isoenzyme aldehyde dehydrogenase II, whereas isoenzyme I was not detected in 39.5% of the alcoholic patients and in 9.5% of those with nonalcoholic liver disease. The lack of aldehyde dehydrogenase I was observed in cases with the lowest enzymatic activities. These results suggest that the decrease of alcohol and aldehyde dehydrogenase activities in alcoholics is not a primary defect and, therefore, their decrease is secondary to liver damage. It is speculated that the diminution of alcohol dehydrogenase, found particularly in alcoholics, could be due to centrilobular cell necrosis. © 1989.