RESUMEN
To identify the evidence supporting the horizontal transmission of hepatitis B virus, HBsAg carrier rate among institutionalised children is determined and compared to children of the same age-group from the well baby clinic, Ramathibodi hospital. The results of this study show that HBsAg is detected four times more frequently in institutionalised children than in children from a well baby clinic. The chance of becoming an HBsAg carrier increases with age and duration of stay in the institution. Repeated study one year later shows that the HBsAg carrier rate among the same group of institutionalised children had increased by 35 per cent. The evidence supports the hypothesis that hepatitis B virus can be transmitted nonparenterally. To prevent this mode of transmission in both institutions and the community, hepatitis B vaccine should be given to these children.