In 1991-92, a cross-sectional survey in Flanders (Belgium) for hepatitis B virus (HBV) markers among 277 relatives of institutionalised mentally handicapped persons to evaluate the prevalence of hepatitis B and the risk of infection showed that relatives of an HBV positive resident were 7.6 times more likely to be infected than relatives of an HBV seronegative mentally handicapped person. HBV infection among the family members could be attributed in 83% to exposure to an HBV positive resident. This study demonstrated the importance of horizontal transmission among relatives of institutionalised mentally handicapped people, even if family contact is reduced to weekend and holiday activities. We recommend that HBV vaccination policy should be expanded to include relatives of institutionalised mentally retarded people.