Objective: Over time, we observed more visits in our pediatric emergency department with length-of-stay (LOS) of more than 10 hours, whereas our mean LOS was approximately 3 hours. We sought to characterize factors associated with this extremely long LOS. Methods: Eighty-one visits with LOS more than 10 hours were identified from January 1, 2001, to June 30, 2003. In this retrospective study, we compared these cases with 405 randomly selected age-matched controls with LOS less than 10 hours (5 controls per case). Results: The groups were similar for sex, visit month, arrival mode, and level of training of the supervising physician. Cases more frequently arrived during night shifts (30% vs 13%) and had laboratory tests (93% vs 32%), radiological studies (83% vs 34%), procedures (28% vs 15%), sedations (24% vs 4%), subspecialty consultations (84% vs 20%), chief complaints of abdominal pain (42% vs 6%) and diagnoses of appendicitis (10% vs 1%), and had a greater hospitalization rate (67 vs 19%). Although more cases involved white patients (57% vs 31%), race was not associated with LOS more than 10 hours in adjusted analysis. In multivariable analysis, longer waiting time (odds ratio [OR], 1.013; 95% confidence interval [CI], 1.007-1.019), night shift arrival (OR, 5.0; 95% CI, 1.9-12.8), higher triage acuity (lowest acuity: OR, 0.003; 95% CI, 0.0-0.286), radiology study other than radiographs (OR, 18.0; 95% CI, 7.5-43.1), and subspecialty consultation (OR, 7.6; 95% CI, 3.2-18.3) were associated with LOS more than 10 hours. Conclusions: In our pediatric emergency department, risk factors for LOS more than 10 hours included longer waiting time, night shift arrivals, high triage acuity, radiology studies, and subspecialty consultations. These factors may also be important considerations for quality improvement initiatives at other institutions.