A cross-sectional survey on the prevalence of hepatitis B serologic markers and hepatitis B virus DNA was performed in a population of 493 mentally handicapped males. Special interest was focused on age-related variables such as age at entry into the institution and on duration of residency. Furthermore, the differences with regard to the prevalence of hepatitis B markers found in Down's syndrome residents and other mentally retarded persons were analyzed. In a longitudinal study, the impact of the presence of hepatitis B virus DNA in serum was studied. Overall, 62.1 per cent of residents had serologic evidence of infection with hepatitis B virus, while 16.7 per cent of those residents with markers of infection were positive for hepatitis B surface antigen (HBsAg). Hepatitis B virus DNA was found in 24 per cent of HBsAg carriers (all positive for hepatitis B e antigen (HBeAg). In residents whose age at entry was less than 15 years, those with Down's syndrome were more often carriers of HBsAg than other mentally retarded residents. In addition, Down's syndrome residents more often had serum hepatitis B virus DNA compared with residents with other forms of mental retardation. A young age at entry was recognized as an important factor with regard to the prevalence of hepatitis B markers. From the longitudinal studies, it appeared that loss of hepatitis B virus DNA from serum indicated imminent loss of HBeAg and normalization of alanine aminotransferase values. Knowledge of the hepatitis B virus DNA status of HBsAg carriers in these institutions may therefore provide a valuable tool in attempts to reduce the transmission of this infection.