OBJECTIVE: The authors determined the seroprevalence of HIV-1 among patients admitted to two psychiatric hospitals in New York City. METHOD: Patients consecutively admitted to an acute psychiatric unit in Manhattan and a large state hospital in Queens were anonymously tested for HIV-1 antibodies from December 1989 through July 1990. Test results were linked to age, gender, ethnicity, and two risk behaviors: male homosexual activity and injection drug use. RESULTS: Blood was obtained from 83.0% of the eligible patients. The prevalence of HIV was 5.5% (25 of 451). Black patients accounted for 38.0% of the patients tested and 76.0% of positive results (N = 19), a rate of 11.1% for this group. The rate of seropositivity was comparable in women and men. Clinicians had charted risk behavior for nine (36.0%) of the 25 HIV-positive patients. Infection control records suggested that clinicians were aware of seven (28.0%) of the positive cases. CONCLUSIONS: One in every 18 patients admitted to two public psychiatric hospitals in New York City was HIV positive. Clinical staff largely failed to identify HIV-positive patients. Ethnicity and a history of homosexual activity among men or use of injected drugs were strongly associated with seropositivity. This pattern of infection may be linked to needle sharing and/or sexual activity with partners who have shared needles. Future research should clarify how psychiatric illness affects risk-taking behavior, focus on improving detection by clinicians, and identify effective prevention strategies in this population.