There has been evidence in the literature suggesting that children with Down's syndrome have a higher incidence of HBsAg than the normal population. This is especially true for children living in institutions. The sera of 30 patients with Down's syndrome and their families were tested for hepatitis associated antigen. These children were routinely seen on out-patient basis, and all of them lived at home with their family. HBsAg was not detected in 66 parents of Down's syndrome patients. Only two children were positive (6.6 %). Our study suggests that in Turkey the frequency of HBs Ag in Down's syndrome patients is considerably higher than in other countries.