In order to determine the prevalence of hepatitis B virus (HBV) markers, 400 patients were studied: 134 residents of an institution (RI) for the mentally retarded and 266 under non-residential care (NRC). In the residential institutions, all markers were absent in 69 (65.7%) of 105 patients with Down syndrome and 20 (69.0%) of 29 clients without Down syndrome. In the NRC clients, 167 (85.6%) of 195 patients with Down syndrome and 65 (91.5%) of 71 clients with other mental defects (psychologically and physically handicapped, autism) had negative tests for HBV markers. The prevalence of the hepatitis B surface antigen (HBsAg) was higher in institutionalized mentally retarded (RI) and older patients (21+ years). Examination of 195 Down patients revealed a higher frequency (1.4 times) of surface antigen carriers as strictly matched non-Down syndrome cases (point prevalences 14.2% and 10.3%, respectively). The higher prevalence in affected cases appears to be primarily associated with a longer persistence of antigenemia. Results related to the sex of the patients were less clear. Neither affected nor non-affected patients showed significant differences in prevalence among males and females. © 1995 Wiley-Liss, Inc.