The prevalence of Helicobacter pylori infection in US adults was shown to be inversely correlated with the socioeconomic status of the family during childhood, and it was suggested that this was additional evidence of transmission occurring in childhood. The present study of H. pylori infection was conducted in South Korea, which has emerged as a developed country in the last two decades. The authors attempted to determine whether there was a difference in prevalence of H. pylori infection in Korean children of different socioeconomic classes despite the high prevalence of infection in childbearing adults. The authors also attempted to identify the factors responsible for the different patterns of transmission by estimating the age-specific prevalence of H. pylori infection in 413 healthy 1- to 75-year-old asymptomatic volunteers who resided in Seoul. H. pylori status was evaluated using an enzyme-linked immunosorbent assay for anti-H. pylori immunoglobulin G. Demographic data were obtained from each individual, and socioeconomic class was assessed by the education level of the adults and of the children's parents as well as family income. H. pylori infection was present in 75% of adults and 22% of children, and its prevalence increased with age (p < 0.001). In adults, the rate of infection was high and independent of socioeconomic class. In children, it was inversely related to the socioeconomic class of the child's family: 12% among upper socioeconomic class, 25% among the middle class, and 41% among the lowest class (p = 0.016). No associations were found between prevalence of H. pylori infection and any factor tested including sex, smoking, and alcohol consumption. In addition, type of housing, whether owned or rented, number of family members living in the same household, water source, and type of community in which a child grew up were not found to be risk factors influencing H. pylori infection prevalence. The prevalence of H. pylori infection in Korea appears to be changing with markedly lower prevalence in children of families of higher socioeconomic status. The factor(s) responsible for the break in the pattern of transmission in children of the higher socioeconomic class was not discovered. Future studies will concentrate on possible differences, eating practices, hygiene, and sanitary practices.