The authors have shown the absence of HBAg in the blood of 30 patients with Down's syndrome treated in ambulatory conditions (age from 2 to 16 years), as well as in 28 brothers or sisters and in 19 of their mothers. After discussing the old concept of relationship between Down's syndrome and HBAg, the authors confirmed the observation that in non-institucionalized patients with the Down's syndrome, the incidence of HBAg is similar to the whole population.