It is now recognized that Helicobacter pylori infection exerts profound and diverse effects on gastric acid secretory function and that the alterations in acid secretion depend on the pattern of gastritis caused by the infection. In patients with an antral predominant nonatrophic gastritis, there is acid hypersecretion leading to duodenal ulcer disease. In patients with an atrophic pangastritis, there is markedly reduced acid secretion and increased risk for gastric cancer. It is now recognized that acid secretion also modifies H. pylori gastritis and a person's premorbid acid secretory status may be an important factor in determining the pattern of gastritis that an individual develops. This two-way interaction between H. pylori gastritis and gastric acid secretion is important in understanding the role of H. pylori infection in the response to proton-pump inhibitor therapy: It explains the more profound control of gastric acid secretion in H. pylori-positive patients and why rebound acid hypersecretion is confined to H. pylori-negative subjects.