In the pathogenesis of gastroesophageal reflux disease (GERD), gastric acid is considered to be one of the most important factors, but little is known about the degree of gastric acid secretion in GERD patients. In this study, we evaluated it in GERD patients and control subjects by 24-h intragastric pH, and serological and histological investigations, in relation to Helicobacter pylori (H. pylori) status. In H. pylori-negative GERD patients gastric acid secretion was similar to that in H. pylori-negative control subjects. In H. pylori-positive GERD patients, in particular, mild GERD patients, it decreased significantly compared to that in H. pylori-negative control subjects, but the degree of decrease was smaller than in H. pylori-positive control subjects. Results of serological and histological evaluation were supportive. In conclusion, in some GERD patients, gastric acid secretion was significantly decreased. Increased or maintained gastric acid secretion was not essential in the pathogenesis of mild GERD.