Helicobacter pylori infection is now recognized to be an important acquired factor in the pathogenesis of duodenal ulcer disease. There is also an association between H pylori and the subsequent development of gastric cancer. The mechanism of the association between the infection and those disorders is incompletely understood but there is increasing evidence that H pylori-induced disturbances of gastric function play a pivotal role. In this article we review the role of H pylori infection in the pathophysiology of these important upper gastrointestinal diseases.