The incidence of gastric cancer and the prevalence of Helicobacter pylori are high in Japan, so it is an important issue whether long-term H. pylori infection leads to chronic atrophic gastritis, considered one of the precursors of gastric cancer. We have reported that the grade of atrophy was higher in H. pylori-positive subjects than in H. pylori-negative subjects. It has also been reported that the atrophy of gastric mucosa increased in H. pylori-infected monkeys compared with control monkeys in a 5-year follow-up study. Most H. pylori infections occur in children, and atrophy of the gastric mucosa progresses during aging. Long-term data show that H. pylori infection can lead to gastric atrophy and may play an important role in the development of gastric cancer. Interestingly, there was no difference in the prevalence of H. pylori between patients with chronic gastritis and gastric cancer in Japan, but the prevalence of H. pylori in young Japanese gastric cancer patients was significantly higher than in the control group. These data clearly show that H. pylori infection is one of the risk factors of gastric cancer in young Japanese people, There is no answer to whether curing H. pylori infection can reverse the atrophy of the gastric mucosa and decrease the risk of gastric cancer development. To clarify this issue, an intervention study must be done. A large clinical trial called the Japanese Intervention Trial of H. pylori is in progress.