The public health implications from H. pylori infection are considerable but the transmission routes are largely unknown. In this study, the prevalence, patient characteristics and risk factors for Helicobacter pylori infection were comparatively investigated in Iceland, Sweden and Estonia. Blood samples were collected from 1046 subjects aged approximately 25-50 y (447 in Reykjavik, 359 in Uppsala and 240 in Tartu) for determination of antibodies to H. pylori and its cagA protein. The prevalence of H. pylori antibodies was 69% in Tartu, 36% in Reykjavik and 11% in Uppsala (p<0.0001). There was an increase in prevalence with age in Iceland and Sweden but not in Estonia. The prevalence of antibodies to the cagA protein in subjects seroreactive to H. pylori was lower in Reykjavik (36%) than in Uppsala (69%) and Tartu (62%) (p<0.0001). H. pylori infection, as determined by seroreactivity, was positively associated with smoking and BMI. Overall, socioeconomic development during the childhood period seems to be the most important factor for the prevalence of H. pylori infection. In adults, smoking may be a contributory factor.