The prevalence of hepatitis B surface antigen (HBsAg) in 155 patients with Down's syndrome (DS) and 209 with other types of mental retardation (OMR) at Huronia Regional Centre, Orillia, Ontario was 34.8 and 5.3%, respectively. There was no significant difference in prevalence between males and females in either group of patients. In 75 matched pairs (DS-OMR) the HBsAg prevalence was 45% in DS and 8.3% in OMR males; in females 40% of those with DS were HBsAg-positive, whereas all the OMR residents were negative. The prevalence of HBsAg in both DS and OMR groups was higher in those admitted in early childhood and in those who had resided in the institution for more than 10 years. In all 54 HBsAg-positive DS patients the antigen subtype was ad. Among the 11 HBsAg-positive OMR patients the subtype was ad in 10 cases and ay in 1.