OBJECTIVE Infection of Helicobacter pylori is viewed as a major driver of progression to the precancerous state or to gastric cancer. This study was performed to investigate the effect of H. pylori infection on gastric cancer development and to determine to what extent H. pylori eradication is likely to reduce the prevalence of gastric cancer. METHODS Gastric cancer development was investigated in 1790 Korean subjects who underwent gastroscopy and H. pylori testing between 1992 and 1998. The effects of H. pylori-positive and eradicated states on gastric cancer development were analyzed. RESULTS Gastric cancer developed in 5 of the study cohort during a mean follow-up period of 9.4 years. All of these patients were positive for H. pylori infection, and 4 of the 5 had antral intestinal metaplasia (IM) at the time of study enrollment. One of these 5 patients was in an eradicated state when the gastric cancer was diagnosed, and had histologic IM before eradication therapy was performed. Gastric cancer was found to develop 10.9 times more frequently in the presence of IM than in its absence. CONCLUSIONS The present study shows a close relationship between H. pylori infection and IM, and between IM and the development of gastric cancer. In addition, our finding suggests that chronic H. pylori infection looks like an important risk factor for the development of gastric cancer in Korea, where the prevalence of H. pylori remains high. This study indicates that to prevent gastric cancer H. pylori eradication is best performed before the development of IM.