To estimate the number of recently infected patients in the window period before human immunodeficiency virus type 1 (HIV-1) seroconversion among patients seeking medical care, randomly selected adults presenting to an inner city emergency department were tested for HIV1 antibody and p24 antigen. Of 2300 patients enrolled, 180 (7.8%; 95% confidence interval [CI]: 6.7%–8.9%) were Western blot (WB)-positive for HIV-1 antibodies. Of 2120 antibody-negative or WB-indeterminate patients, none of whom were identified on clinical grounds as having primary HIV-1 infection, 6 (0.28%; CI, 0.07%–0.51%) were p24 antigen-positive with serologies consistent with primary HIV-1 infection. Of these 6, 3 were seronegative even with third-generation antibody ELISA. HIV-1 infection in these 6 patients was further confirmed by polymerase chain reaction amplification of virion-associated RNA in serum demonstrating 10⁴–10⁵ virions/mL. With 40,000 new HIV-1 infections in the United States annually, ∼750 persons with undiagnosed primary HIV-1 infections may seek primary health care in any given week in the United States. Testing for viral antibodies alone will fail to detect a large proportion of these persons. Thus, early identification by p24 antigen testing may be important to diagnose and treat symptomatic illness, implement public health and counseling measures, and arrange appropriate medical follow-up.