Screening for HIV in Emergency Departments (EDs) is recommended to address the problem of undiagnosed HIV. Serosurveys are an important method for estimating the prevalence of undiagnosed HIV and can provide insight into the effectiveness of an HIV screening strategy. We performed a blinded serosurvey in an ED offering non-targeted HIV screening to determine the proportion of patients with undiagnosed HIV who were diagnosed during their visit. The study was conducted in a high-volume, urban ED and included patients who had blood drawn for clinical purposes and had sufficient remnant specimen to undergo deidentified HIV testing. Among 4752 patients not previously diagnosed with HIV, 1403 (29.5%) were offered HIV screening and 543 (38.7% of those offered) consented. Overall, undiagnosed HIV was present in 12 patients (0.25%): six among those offered screening (0.4%), and six among those not offered screening (0.2%). Among those with undiagnosed HIV, two (16.7%) consented to screening and were diagnosed during their visit. Despite efforts to increase HIV screening, more than 80% of patients with undiagnosed HIV were not tested during their ED visit. Although half of those with undiagnosed HIV were missed because they were not offered screening, the yield was further diminished because a substantial proportion of patients declined screening. To avoid missed opportunities for diagnosis in the ED, strategies to further improve implementation of HIV screening and optimize rates of consent are needed.