Lactase and sucrase activities were measured in jejunal biopsies from a group of alcoholic and nonalcoholic men of similar nutritional status, consisting of American blacks, and whites of northern European origin. When measured withing 10 days of alcohol withdrawal, sucrase activity was decreased by 33% in the alcoholics. Lactase activity was less than 1 U per g in 100% of the black and 20% of the white alcholics as compared to 50% of the black and none of the white control subjects. Lactase activity was virtually absent in 45% of the black alcoholics. A second jejunal biopsy after an additional 2-week period of alcohol abstinenece exhibited significant secondary increases in the activities of both disaccharidases. Oral administration of lactose (1 g per kg of body weight) resulted in significantly lower blood glucose concentration and higher incidence of adverse effects in alcoholics, mainly among the blacks. Although data from larger populations are needed to confirm our observations, these findings suggest that chronic alcohol ingestion decreases intestinal disaccharidase activities even in the absence of overt malnutrition. The decrease in enzyme activity produced by alcohol is associated with increased morbidity after lactose administration.