Although mortality rates owing to gastric cancer have gradually decreased, the morbidity rates are still high in Japan. Helicobacter pylori(H. pylori)infection is an important risk factor for gastric cancer, although most individuals with an H. pylori infection do not develop this malignancy. Therefore, it is speculated that other risk factors including lifestyle contribute to the enhanced risk of gastric cancer posed by H. pylori infection. It is also noteworthy that the prevalence of prediabetes and diabetes has dramatically increased in Japan, and cancer is now considered a possible complication of the latter. However, there have been very few epidemiological studies evaluating the relationship between diabetes and gastric cancer in Japan. The Hisayama Study, which is a prospective cohort study conducted in a Japanese community, demonstrated that the incidence of gastric cancer significantly increased with elevated fasting plasma glucose levels. A modest increase in hemoglobin A1c levels was also a significant risk factor for gastric cancer even after adjusting for other risk factors, including H. pylori infection. Moreover, among subjects who had both a high hemoglobin A1c level and H. pylori infection, the risk of gastric cancer increased synergistically. These findings suggest that even prediabetic hyperglycemia is a significant risk factor for gastric cancer in the general Japanese population. Early identification of hyperglycemia and an appropriate behavioral and therapeutic approach may help prevent gastric cancer in Japan where there is a high incidence of both this malignancy and diabetes.