A study was undertaken to determine the extent to which physician assistants (PAs) and/or nurse practitioners (NPs) are a source of health care delivery in emergency departments (EDs) in the United States. The National Hospital Ambulatory Medical Survey (NHAMCS) uses a multistage probability sample that examines patient visits within EDs. The sample included 437 hospitals with EDs. Visits were mostly from self-referred patients to EDs within nonfederal, short-stay hospitals, or general hospitals. Analysis of NHAMCS data found that a PA and/or NP was seen for 3.5 million ED visits in 1992. Remarkably little difference in gender, reason for visit, diagnosis, and medication prescribed was found between PA/NP visits and visits to all providers. This was the first study that systematically identified the extent of PA/NP-delivered ED services in the United States and compared it with physician services. Overall, PAs and NPs were found to be significant sources of health care service for hospital EDs. They are involved in care for almost 4% of all ED visits nationally and manage a wider range of conditions than has been previously reported. When types of visits are analyzed, including reasons for ED care, diagnosis, and treatment, it appears that visits associated with care by ED-based PA/NPs are similar to all ED visits, including those attended by emergency medicine physicians. More studies are needed to better understand the role of PAs and/or NPs in various ED settings. Recruitment and use of PAs and NPs may be a cost-effective strategy for improved delivery of emergency services.