Background/Aims: The aims of this study were to investigate the seroprevalence of H. pylori infec-tion and to find out the various epidemiologic factors related to H. pylori infection in Korea. Methods: From May, 1997 to July, 1997, 2,449 healthy subjects from 5 health center responded to the self- administered questionnaires for various epidemiologic factors. At the same time, H. pylori IgG level in serum were tested by ELISA (GAP test) and proved to be H. pylori 'positive' in cases whose cut-off values were over 15 μ/mL. Results: The overall seroprevalence of H. pylori infection was 44.8%. In relation to age, the seroprevalence of H. pylori infection was 57.8% in adults (age>18), and 15.3% in children (age, 1-18). The seroprevalence was 1.1% in younger group than 5, 12.8% in group aged between 5 and 9, 20.4% in group aged between 10 and 14, 33.3% in group aged between 15 and 19, and 66.7% in the 20' s. The results mean that the prevalence increases with age (p<0.001). Besides age, the other significant epidemiological factors affecting the seroprevalence of H. pylori infection were occupation, water source, presence of gastrointestinal symptoms in adults and FH of PUD and number of family members in children. Conclusions: In this study, the seropre-valence of H. pylori infection in adults was 57.8% which is lower than that of previous reports Further epidemiologic studies is needed to identify the role of epidemiologic factors of childhood and adolescent period, the major affected periods. (Kor J Gastroenterol 1999;33:170 - 182)