The morphology of the small-intestinal mucosa was studied in 11 alcoholic patients admitted to hospital for detoxification. A first biopsy specimen from the small intestine was taken as soon as possible after admission and a second specimen after about 6 weeks of abstinence. The specimens were studied in the light microscope, in the scanning electron microscope, and by immunohistochemistry. Morphometrically, a slight reduction in villus height in relation to crypt depth was observed. One patient had a subtotal villus atrophy. After abstinence the villus height was increased in five of the six patients who accepted a second biopsy. No obvious changes were seen in the frequency and appearance of peptidergic nerves or endocrine cells. Ultrastructurally, pronounced alterations were seen in the surface ultrastructure of the enterocytes. In two specimens bacterial adhesion to the mucosal surface was also found. The ultrastructural changes were unaltered after abstinence. In serum the concentrations of zinc were reduced, and the levels of copper were elevated compared with a group of teetotallers. A recently developed marker of high alcohol consumption, carbohydrate-deficient transferrin, was as good an indicator as the other conventional biochemical markers.