The aim of this study was to investigate whetherchronic alcohol abuse alters the number and distributionof mononuclear cells of the duodenal mucosa. The numberof common leukocyte antigen (CLA)-positive interepithelial lymphocytes (IEL), Blymphocytes (BL), IgAproducing plasma cells (IgA-PC),and macrophages (MP) was quantitatively evaluated inbiopsies of the duodenal mucosa of patients with alcoholabuse compared to subjects without alcohol abuse.Biopsies from the descending part of the duodenum wereobtained by endoscopy from two groups of patients withchronic alcohol abuse (group A1, abstinence <5 days, N = 21) and group A2 abstaining 5-10 days(N = 6). Twenty-five subjects without alcohol abuseserved as controls (C). Immunohistochemical staining wasdone by avidinbiotin- complex method. In addition, the content of IgA in the plasma cells wasdetermined by using a TV-densitometric method. Thenumber of B-lymphocytes in the lamina propria wasincreased by 37% in group A1 (P < 0.005). A distinctdecrease was observed in group A1 compared to C in thenumber of IEL that were CLA positive (–50%, P <0.025) and in the number of macrophages (–54%, P< 0.025). In group A2 the differences in the number of B lymphocytes and macrophages were no longerseen. In A1, there was no significant change in thenumber of IgA-producing plasma cells or in the number ofinterepithelial lymphocytes counted after H&E staining compared to the controls. There was nodifference in content of IgA in the IgA-producing plasmacells. From these results it is concluded that chronicalcohol abuse significantly influences the gut-associated immune system, possibly byincreasing the permeability of the gut mucosa tomacromolecules that act as antigens.