In this study, we found Lewis X (Le x ) determinants on 68% of Helicobacter pylori isolates from patients with chronic gastroduodenal diseases. Anti-Le x IgG were detected more frequently in the sera from dyspeptic children and adults (45 and 46%), with or without proved (culture) H. pylori infection, than in the sera from healthy individuals (14% and 25%). In contrast, the prevalence of anti-Le x IgM was higher in the groups of healthy individuals than in the groups of dyspeptic patients. Moreover, anti-Le x monoclonal antibody of IgM class enhanced the uptake of Le x (+) but not Le x (-) H. pylori isolates by phagocytes. In the sera from some dyspeptic patients, we detected Le x -anti-Le x IgG immune complexes (Le x ICs). There was a great difference between children and adults as regards the presence of Le x ICs. The immune complexes were found in the sera from nine out of 29 (27%) H. pylori-infected and three out of eight (37%) uninfected adult dyspeptic patients. In comparison, Le x -anti-Le x IgG ICs were detected only for two out of 18 (11%) H. pylori-infected children. Le x ICs were not found in the sera from healthy individuals. Our results suggest that anti-Le x IgM may play a protective role in H. pylori infections. In contrast, anti-Le x IgG and particularly Le x -anti-Le x IgG ICs might contribute to the pathogenesis of chronic H. pylori infections.