Prevalence of anti-HCV antibody in 1193 hospitalized patients (848 males and 345 females) in psychiatric department was investigated. Overall prevalence ratio was 9.1%, indicating significantly higher than that of healthy blood donors. In the classification of ICD-10, the prevalence of the inpatients diagnosed in schizophrenia group and psychoactive substance use group accounted for 6.2% and 13.8%, respectively. However, adequate reasons such as sanitary issues were not found to account for the high prevalence. Only the age of the patients could account for the high prevalence in the schizophrenic group. In the psychoactive substance abuse group, the sanitary issues might be a major cause of the very high prevalence of anti-HCV antibody, while other factors such as dysfunction of the immune system might be considered to account for it.