This article describes a study undertaken to determine the prevalence of human immunodeficiency virus (HIV) infection and risk factors associated with HIV infection in a chronically mentally ill population. Patients were eligible for inclusion in the survey at their first outpatient or inpatient admission to a Maryland state psychiatric hospital between August 1990 and July 1991. Demographic and risk behavior information was collected, and unlinked HIV antibody testing was performed on blood specimens drawn for routine clinical purposes. Of 533 patients surveyed, 31 patients (5.8%) were infected with HIV. The prevalence of HIV infection was 36.4% among female patients reporting intravenous drug use and 14.5% among their male counterparts. The prevalence of HIV infection among homeless patients was 10.1%; 88.9% of those HIV infected also reported intravenous drug use. On admission, 90% of patients reported no knowledge of their HIV antibody status; 4.1% of these patients were HIV infected. The data confirmed the risk of HIV infection in this population of chronically mentally ill individuals. Risk reduction programs designed specifically for individuals with chronic mental illness need to be developed.