Objective: To compare pediatric mental health emergency department visits to other pediatric emergency department visits, focusing on length of stay. Method: We analyzed data from the National Hospital Ambulatory Medical Care Survey, a nationally representative sample of US emergency department visits from 2001 to 2008, for patients aged less than or equal to 18 years (n = 73,015). Visits with a principal diagnosis of a mental disorder (n = 1,476) were compared to visits (n = 71,539) with regard to patient and hospital characteristics, treatment, and length of stay. Predictors of prolonged mental health visits were identified. Results: Mental health visits were more likely than other visits to arrive by ambulance (21.8% versus 6.3%, p less than 0.001), to be triaged to rapid evaluation (27.9% versus 14.9%, p less than 0.001), and to be admitted (16.4% versus 7.6%, p less than 0.001) or transferred (15.7% versus 1.5%, p less than 0.001). The median length of stay for mental health visits (169 minutes) significantly exceeded that of other visits (108 minutes). The odds of extended stay beyond 4 hours for mental health visits was almost twice that for other visits (adjusted odds ratio 1.9, 95% CI = 1.5-2.4) and was not explained by observed differences in evaluation, treatment, or disposition. Among mental health visits, advancing calendar year of study, intentional self-injury, age 6-13 years, Northeastern, Southern, and metropolitan hospital location, use of laboratory studies, and patient transfer all predicted extended stays. Conclusions: Compared with other pediatric emergency visits, mental health visits are longer, are more frequently triaged to urgent evaluation, and more likely to result in patient admission or transfer, thereby placing distinctive burdens on US emergency departments. (Contains 4 tables and 1 figure.)