Hepatitis A is an infectious disease commonly found in many developing countries. The infection usually is asymptomatic in children and only a small percentage has clinical hepatitis of varying severity. Hepatitis A infection becomes more symptomatic with increasing age. We studied hepatitis A antibody prevalence in many population groups of Thailand and investigated the changing patterns of hepatitis A antibody in comparison with previous seroepidemiological surveys in the past decade. The antibody prevalence in Bangkok and in the rural areas of Thailand had markedly declined among the children and adolescents studied. Only one-fifth to one-fourth of children and adolescents had antibody to hepatitis A. The prevalence differed from that of a decade ago. These data probably reflect the marked improvement of hygienic conditions and effective health education, which have reduced exposure to HAV infection among young children. When an effective hepatitis A vaccine with long lasting immunity becomes available, subgroups of the populations at risk of infection will be the targets for immunization.