Background: Gastric-acid secretion is reduced in Helicobacter pylori-positive fundic atrophic gastritis, but it is restored by the eradication. However, changes in the distribution of acid-secreting mucosa after the eradication remain unknown. Congo red chromoendoscopy is capable of visualizing the acid-secreting fundic mucosa. Objective: To evaluate the effect of H pylori eradication on the distribution of acid-secreting mucosa in the fundus by using Congo red chromoendoscopy. Design: An assessment of the distribution of acid-secreting mucosa by the visualized images of Congo red chromoendoscopy and a histologic evaluation of biopsy specimens were performed before and 1 and 7 months after the eradication. The areas of the acid-secreting mucosa in the lesser and greater curvatures of the fundus were evaluated semiquantitatively. Patients: Thirty-seven patients positive for H pylori and with fundic atrophic gastritis. Results: The area of the acid-secreting mucosa increased in 27 cases (73%) by 1 month after eradication, and in 32 cases (86%) by 7 months. This expansion of the acid-secreting mucosa coincided with the improvement of inflammatory changes rather than with that of the mucosal atrophy and was more prominent on the greater curvature than on the lesser curvature. Conclusions: The acid-secreting mucosa in the fundus expanded in most cases with fundic atrophic gastritis after H pylori eradication, which could be responsible for the increase in acid secretion after the treatment. Regional differences in the recovery of local acid secretion may be associated with site-specific susceptibility to the development of gastric cancer after successful eradication.