Seroprevalence for HIV-1 was anonymously evaluated between November 1989 and July 1991 among severely mentally ill patients at two public psychiatric hospitals in New York City. The study population consisted of new admissions and long-stay patients aged 18-59. Of 1116 eligible patients, usable samples were obtained from routine blood drawings on 971 (87%). Seroprevalence was comparable among men (5.2%) and women (5.3%). Age did not predict seropositivity. Men with a recorded history of homosexual behaviour or injection drug use were, respectively, 1.8 and 2.0 times more likely to be seropositive than men without these histories. Women with a recorded history of injection drug use were 4.0 times more likely to be seropositive than women without such a history. Ethnicity was not predictive for men, but Black women were 2.4 times more likely to be HIV-1 positive than non-Black women. Severely mentally ill inpatients had a substantial rate of HIV-1 seropositivity, indicating a need for additional testing, education and counselling efforts for this population.