To investigate the seroprevalence of hepatitis B surface antigen (HBsAg) and antibodies to hepatitis C virus (anti-HCV) in a psychiatric institution in Taiwan, where hepatitis B virus (HBV) is hyperendemic, a total of 780 patients with psychiatric disorders were studied. Enzyme-linked immunosorbent assays (ELISA) were used for testing HBsAg and anti-HCV. The prevalence of HBsAg was higher than that of anti-HCV among these patients (18.1% vs. 6.8%, P < 0.0001). The HBsAg carrier rate in these patients was consistent with that of the general population, with a trend for HBsAg carrier rate to be lower in the aged and in females. In contrast, the prevalence of anti-HCV was higher in these patients than in general population. Anti-HCV positivity was found more frequently in patients who had received blood transfusion previously (24% vs. 6.4%, P < 0.05). The majority (92%) of patients with positive anti-HCV did not have a history of apparent parenteral exposure. The prevalence of anti-HCV increased significantly with duration of the psychiatric disorder. The prevalence of anti-HCV also tended to increase with duration of hospitalization but without reaching statistical significance. These findings suggest that these institutionalized psychiatric patients contract hepatitis B, as does the general population in Taiwan, and they should be considered as a specific risk group for hepatitis C infection.