Statement of Problem: The Prevalence of HBsAg, among patients with Down Syndrome (DS) has been reported higher than normal population. Living in institutes, comparing to home, leads to higher probability of HBsAg prevalence.

Aim: The purpose of the present study was to determine HBsAg prevalence in patients with DS and compare it with other mentally retarded patients with similar conditions, and also with healthy population in Iran.
Materials and Methods: One hundred patients, with average age of 12 years, living in five institutes inTehran, were chosen. Children with DS and other mentally retarded children were considered as the first and second groups, respectively (50 members in each group). Blood samples were taken and tested (ELLSA) to determine HBsAg and HBsAb titre. For statistical analysis, Chi- Square test was used.

Results: HBsAg was positive in 8% and 4% of DS and MR groups, respectively. HBsAb was seen in 25% of DS, comparing to 32% of MR. The prevalence of HBsAg in retarded patients living in institutes was higher than normal population of Iran (less than 3%), which was not statistically significant.
Conclusion: Consideration of sterilization principles and infection control upon dental operations, in these centers, are necessary. It is also suggested that these children, as a high- risk population, should be vaccinated against hepatitis B.