We compared the seroepidemiologic patterns of Helicobacter pylori and hepatitis A virus (HAV) infections among participants in 2 independent cross-sectional studies conducted in Japan in 1986 and 1994. Subgroups were monitored with successive blood sampling. H. pylori and HAV infection status was defined by results of enzyme-linked immunosorbent assay. In 1986, the prevalence of H. pylori infection and HAV infection, respectively, were 80% and 70% among adults and 31% and 5% among children. The prevalence of both infections increased with age. Concordant infections were found in 74.5% of adults (kappa=0.2) versus 2% of children (kappa=0.05). During the 9-year study period, the incidence of H. pylori infection was 1.1% among adults and 2% among children. The seroprevalence of HAV remained constant. The disparity between the increase in prevalence of H. pylori and HAV infection with age is likely associated with improvements in hygienic practices. The discordance between the presence of the infections among younger persons is evidence against a common source and/or vehicle for transmission.