Recent studies on the relationship between Helicobacter pylori and gastritis or gastroduodenal ulcers have indicated that this organism can induce chronic active gastritis with neutrophil infiltration. However the mechanisms by which bacteria colonized and damage the host are often complex and multiple. The ammonia produced by H. pylori, which has strong urease activity, has been reported to have a cytotoxic effect on the gastric mucosa. With H. pylori the major areas of interest regarding pathogenesis are also related to other factors such as oxygen radicals, cytotoxins and cytokines. There may also be virulence differences between H. pylori strains which may determine the severity of the gastric mucosal damage. Clearly this is an important area for future research.