OBJECTIVE: To estimate the prevalence of and to identify factors associated with prolonged emergency department length-of-stay (ED-LOS) for admitted children. METHODS: Data were from the 2001-2006 National Hospital Ambulatory Medical Care Survey. The primary outcome was prolonged ED-LOS (defined as total ED time >8 hours) among admitted children. Predictor variables included patient-level (eg, demographics including race/ethnicity, triage score, diagnosis, and admission to inpatient bed vs intensive care unit), physician-level (intern/resident vs attending physician), and system-level (eg, region, metropolitan area, ED and hospital type, time and season, and diagnostic and therapeutic procedures) factors. Multivariable logistic regression was performed to identify independent predictors of prolonged ED-LOS. RESULTS: Median ED-LOS for admitted children was 3.7 hours. Thirteen percent of pediatric patients admitted from the ED experienced prolonged ED-LOS. Factors associated with prolonged ED-LOS for admitted children were Hispanic ethnicity (odds ratio [OR], 1.76; 95% confidence interval [95% CI], 1.10-2.81), ED arrival between midnight and 8 a.m. (OR, 2.80; 95% CI, 1.87-4.20), winter season (January-March: OR, 1.81; 95% CI, 1.20-2.74), computed tomography scan or magnetic resonance imaging (OR, 1.65; 95% CI, 1.05-2.58), and intravenous fluids or medications (OR, 1.81; 95% CI, 1.10-2.97). Children requiring ICU admissions (OR, 0.29; 95% CI, 0.11-0.77) or receiving pulse oximetry in the ED (OR, 0.52; 95% CI, 0.34-0.81) had a lower risk of experiencing prolonged ED-LOS. CONCLUSIONS: We found that prolonged ED-LOS occurs frequently for admitted pediatric patients and is associated with Hispanic ethnicity, presentation during winter season, and early morning arrival. Potential strategies to reduce ED-LOS include improved availability of interpreter services and enhanced staffing and additional inpatient bed availability during winter season and overnight hours.