Personnel of inner-city emergency departments (EDs), which are frequently the only source of medical care for many patients, may be in a unique position to detect human immunodeficiency virus (HIV) infection earlier than personnel at other recommended screening sites. To assist development of ED-based screening strategies for HIV infection, we undertook a serosurvey of HIV infection in adult patients attending an ED during a 6-week period in 1992 using an identity-unlinked technique and compared our findings with data collected similarly in 1988. Of 1,606 patients, 183 (11.4%) were HIV-positive, compared with 6.0% in 1988. Seroprevalence rates of HIV infection among patients only at risk of heterosexual transmission increased more than fourfold (7% to 30.3%). CD4+ cell counts were higher in those patients with undiagnosed HIV infection than in those with known HIV infection. Targeting minority patients aged 25-44 years, intravenous drug users, and those patients at heterosexual risk would have identified 87% of patients with new HIV infection, while requiring screening of 41% of the study sample. Targeted voluntary screening programs in certain EDs would likely detect significant numbers of new early HIV infections.