Hepatitis A virus infection constitutes a world-wide public health problem, predominantly in developing countries. Mentally handicapped children, due to their incapacity for looking after themselves, comprise one of the high risk groups for hepatitis A virus infection. The aim of the present study was to determine the prevalence of hepatitis A virus antibody (anti-HAV) among the children and adults in the Institute for the Mentally Handicapped located in Nonthaburi, Thailand. The prevalence of anti-HAV IgG antibody was 92%. Immunity acquired against HAV was shown to increase in direct proportion to the age. To prevent future outbreaks of hepatitis A, water supply, sanitary conditions and personal hygiene should be improved at this and similar institutions. Furthermore, persons new to the institution (patients and staff) should be screened for anti-HAV and vaccinated with hepatitis A vaccine if nonimmune.