Background/Aims : Helicobacter pylori (H. pylori) infection and gastric adenocarcinoma are highly prevalent in Korea. Thus, it is suspected that H. pylori infection is associated with the development of gastric cancer through the process of glandular atrophy and intestinal metaplasia. We have investigated the effects of H. pylori eradication on gastric histology using updated Sydney system of gastritis. Methods: One hundred thirty-three patients with peptic ulcer associated with H. pylori infection had received H. pylori eradication therapy for 1 week (PPI based triple therapy). Then, the patients were investigated by endoscopy with gastric antral biopsy at 3 month (n=133) and 9 month (n=81) after completing eradication therapy. Results: The eradication rate was 84.9% (113/133). In H. pylori eradicated patients, the neutrophilic activity and chronic inflammation were markedly improved at 3 month (p<0.05). However, the glandular atrophy and intestinal metaplasia remained unchanged until 9 months. Reinfection rate with H. pylori at 9 month was 13.6% (11/81), and most of these were suspected as recrudescent. Thus, real natural reinfection rate was thought to be much lower. Conclusions : H. pylori eradication showed no effect on the glandular atrophy and intestinal metaplasia during follow-up of 9 months. (Kor J Gastroenterol 1999;34:572 - 581)