Background : The prevalence of Helicobacter pylori (H. pylori) infection varies between countries and between social classes. The aim of this study was to identify risk factors for with Helicobacter pylori infection in asymptomatic Korean population. Methods : Sera were collected from 2,687 females and 3,049 males (mean age, 29.1 y; range, 1 m-79 y) in Korea from Mar 1998 through Oct 1998. All asymptomatic subjects completed assessment questionnaires. An enzyme-linked immunosorbent assay was performed to detect IgG antibody to H. pylori.Results : The overall seroprevalence observed was 46.6% and showed no statistical diffrerence between female (45.9%) and male (47.2%). The seroprevalences in children (neonate-15 y) and adult (16-79 y) were 17.2% and 66.9%, respectively. According to multivariate analysis, variables such as sex, age, geographic area, crowding (number of person per room) in childhood, economic status in childhood, and types of housing in childhood were significantly and independently associated with H. pylori seroprevalence of adults. In children, age, geographic area, income, mother's education, and drinking water source were significant risk factors of H. pylori infection. Conclusion : Socioeconomic condition and close person to person contact in childhood are the significant determinants for H. pylori infection in adult. Drinking water source is an another important risk factor for H. pylori infection in children, suggesting the fecal to oral transmission in Korea.(Korean J Med 59:376-387, 2000)