From September 1988 to August 1989, in a university hospital in Newark, NJ, 3529 serum and plasma specimens from patients with admitting conditions presumably not associated with human immunodeficiency virus (HIV) infection (Centers for Disease Control, Atlanta, Ga, Sentinel Hospital Surveillance System criteria) were tested anonymously for the presence of type 1 HIV (HIV-1) antibody. Of these specimens, 269 (7.6%) were confirmed HIV-1 seropositive. Overall, 10.3% of male patients and 4.8% of female patients were seropositive. Persons 25 to 44 years old had the highest HIV-1 seroprevalence- 20.9% for male and 7.5% for female patients. Based on this anonymous testing, the number of HIV-infected hospitalized patients discharged in 1988 was estimated. Data on hospital-confirmed HIV-infected patients tested on the basis of clinical suspicion suggest that only 40% of HIV-infected patients were actually tested for HIV-1 infection as part of their medical care in this hospital. These data demonstrate a high prevalence of HIV infection in this patient population and suggest that hospitals serving populations with a high HIV seroprevalence offer routine screening for HIV infection as part of good medical care.