AIMS: To study the expression of HLA-DR antigen in the different histological types of gastric polyps. METHODS: Ninety five cases of gastric polyps were histologically classified and examined for the presence of Helicobacter pylori, and for degree and type of inflammation. Further sections were stained immunohistochemically for HLA-DR antigen expression in the epithelium using a monoclonal antibody that was reactive to formalin-fixed paraffin wax embedded tissue. RESULTS: HLA-DR antigen was expressed in all of the inflammatory polyps studied (20/20), and in most hyperplastic (12/16) and adenomatous (4/6) polyps. Only a few fundic gland polyps (8/51) stained positively for HLA-DR antigen. Gastric polyps seem to have a greater tendency to express HLA-DR antigens than non-polypoid gastric mucosa, even after considering the factors that may affect HLA-DR antigen expression, such as inflammation and the presence of H pylori. CONCLUSIONS: Growth disturbances/polyp formation may be associated with increased HLA-DR antigen expression.