Background: The role of acid secretion in reflux oesophagitis which may develop after H. pylori eradication is not well known. Aim: To investigate the participation of altered gastric acid secretion and the presence of hiatal hernia in the development of reflux oesophagitis after eradication therapy for H. pylori. Subjects and Methods: A total of 105 patients with H. pylori infection, but without reflux oesophagitis at the time of eradication therapy, were followed prospectively for 7 months after the clearance of this microorganism. Gastric acid secretion was assessed by endoscopic gastrin test, and the presence of hiatal hernia by endoscopy. Results: Reflux oesophagitis developed in 11 out of 105 (10.5%) patients when examined at 7 months after the eradication therapy. The incidence was correlated significantly with the increase in gastric acid secretion after the eradication of H. pylori, and was significantly higher in the patients with hiatal hernia (20%) than in those without it (0%). Conclusions: Increased acid secretion after H. pylori eradication is an important risk factor of reflux oesophagitis, especially in patients with hiatal hernia.