Objectives: The median emergency department (ED) boarding time for admitted patients has been a nationally reportable core measure that now also affects ED accreditation and reimbursement. However, no direct national probability samples of ED boarding data have been available to guide this policy until now. The authors studied new National Hospital Ambulatory Medical Care Survey (NHAMCS) survey items to establish baseline values, to generate hypotheses for future research, and to help improve survey quality in the future.Methods: This was a cross-sectional, multistage, stratified annual analysis of EDs and ED visits from the National Hospital Ambulatory Medical Care Survey public use files from 2007 to 2010, a total of 139,502 visit records. These data represent the only national measure of ED boarding. The main outcome of interest was boarding duration for individual patient visits. Data analyses accounted for complex sampling design.Results: The national median boarding time was 79 minutes, with an interquartile range of 36 to 145 minutes. The prevalence of boarding for more than 2 hours among admitted patients was 32% (95% confidence interval [CI] = 30% to 35%). Average ED volume, occupancy, acuity, and hospital admission rates increased abruptly from the second to the third quartile of median boarding duration. The half of hospitals with the longest median boarding times accounted for 73% of ED visits and 79% of ED hospitalizations nationally. Thirty-nine percent of EDs (95% CI = 32% to 46%) reported never holding patients for more than 2 hours, but visit-level analysis at these EDs found that 21% of admissions did in fact stay in the ED over 2 hours. Only 19% of EDs (95% CI = 16% to 22%) used a strategy of moving admitted patients to alternative sites in the hospital during crowded times. Conclusions:In this national survey, ED boarding of admitted patients disproportionately affects hospitals with higher ED volumes, which also see sicker patients who wait longer to be seen, but not hospitals with higher proportions of Medicaid or uninsured visits. This finding implies that, unlike other quality measures, there is a negative volume-outcome relationship for timely hospitalization from the ED.ACADEMIC EMERGENCY MEDICINE 2014;21:497-503 © 2014 by the Society for Academic Emergency Medicine I ncreasing emergency department (ED) visits have caused rising apprehension in the medical community for over half a century, 1 and the trend in ED crowding continues unabated.2 A 2009 U.S. Government Accountability Office (GAO) report concluded that the most important cause of ED crowding is the lack of access to inpatient beds, leading to prolonged ED stays for admitted patients, often in ED hallways.3 These patients are considered to be "boarding" in the ED. Since the GAO report, there have been several