Epidemiological evidence has shown that seropositivity for Helicobacter pylori in the stomach is positively correlated with the incidence of gastric cancer. In this study, we investigated serum IgG antibody against H. pylori in asymptomatic Japanese subjects by an enzyme-linked immunoassay (ELISA) method (Serion, Wuerzburg, Germany). We noted two characteristics: (1) Seropositivity was seen in 41% of individuals aged less than 1 year, and in 9% of those aged 1-2 years, indicating the possibility that IgG antibody in infancy is derived from the mother. (2) Seropositivity was seen in 35% of individuals aged 15-19 years compared to 70% in those aged 20-24 years, indicating the spread of H. pylori infection with age.