Changes in Helicobacter pylori seroprevalence were studied determining IgG and IgA antibodies of 408 randomly selected adults aged 15-74 years living in Vammala, Finland in 1973 and of 504 similarly selected subjects in 1994. Seroprevalence increased by age at both time points. The age-adjusted seroprevalence rate was clearly lower in 1994 than in 1973 (31 vs. 56%, P = 0.001). Paired serum samples of 224 subjects collected in 1973 and 1994 showed that the antibody status remained unaltered in 92%; 4% seroconverted and 4% seroreverted within the 21 years. The decrease in the seroprevalence rate in the population and the persistence of individual antibody status over two decades support a difference in H. pylori infection rates among birth cohorts over time rather than continuous acquisition of new infections with advancing age. Thus the risk of helicobacter infection in Vammala, Finland has been highest in childhood and continuously decreased at least for the last five decades.