Emergency departments (EDs) provide round-the-clock emergency care but also serve as a health care "safety net." We seek to determine the number, distribution, and characteristics of US EDs, with a long-term goal of improving access to emergency care. We created an inventory of nonfederal nonspecialty US hospitals using 2001 data from 2 independent sources. Hospitals that did not report ED visit data, or with large changes in visit volume by 2003, were contacted to obtain or verify visit volume (n=437; 9% of all hospitals). EDs were divided into 2 groups: those with at least 1 patient per hour, 24 hours per day, 7 days per week (> or = 8,760 visits/year) and those with fewer visits. Of 4,917 hospitals, 4,862 (99%) reported an ED. These EDs collectively received 101.6 million visits. One in 3 EDs (n=1,535) received less than 8,760 visits per year; the national median was 15,711 visits per year. Excluding the low-volume EDs, the remaining 3,327 reported 95.2 million annual visits. The typical higher-volume ED received approximately 28,000 visits per year; 28% (n=922) were in a nonurban setting. Among all EDs, per-capita visits varied by state, with the highest ED visit rates in Washington, DC; West Virginia; and Mississippi. Significant variation exists in the distribution and use of US EDs. One third of EDs have an annual visit volume less than 8,760 and, together, they account for 6% of all visits. The United States should consider classifying EDs, as it does trauma centers, to clarify the type of care available in this heterogeneous clinical setting and the distribution of different types of EDs.