Until recently, gastric acid secretion has been believed to decrease according to age. Previously the atrophy of gastric mucosa that was the main cause for the decrease in acid secretion was understood as the phenomenon following aging. However, H. pylori was discovered and the infection was indicated to be the main cause for the atrophic change of gastric mucosa. In recent studies, gastric acid secretion has been indicated not to decrease in old people who have no atrophic change of gastric mucosa and the infection. Furthermore, some studies indicated an increase in gastric acid secretion in the old people compared with young people. Consequently the decrease in acid secretion according to aging is now thought to be the result of H. pylori infection and not the result of physiological aging.