Helicobacter pylori infection exerts variable effects on gastric acid secretion. It may increase acid secretion, decrease acid secretion or produce no overall change. The effect of the infection on acid secretion depends upon the relative extent to which the Helicobacter pylori gastritis affects the antral and body mucosa of the stomach. When there is antral predominant, body-sparing gastritis, there is increased gastrin release and this is accompanied by increased acid secretion. When there is a significant body gastritis, acid secretion is reduced and subjects may be completely achlorhydric. The majority of subjects have both antral gastritis and body gastritis and this results in no overall change in gastric acid secretion. There is now increasing evidence that the alteration which Helicobacter pylori infection exerts upon gastric acid secretion is a pivotal factor in determining the clinical outcome of the infection. Subjects in whom the infection produces acid hypersecretion develop duodenal ulcer disease due to the increased duodenal acid load. In subjects in whom the infection induces marked hypochlorhydria, there is an increased risk of gastric cancer. The hypochlorhydria probably plays an important role in the carcinogenic process as high intragastric pH markedly raises intragastric nitrite levels, profoundly lowers gastric juice ascorbic acid and allows colonization by nitrosating bacteria. The reason for the different functional responses to Helicobacter pylori infection is unclear but may be related to the host's pre-morbid acid secretory status.