Designing robust clinical trials in critically ill, mechanically ventilated children requires an understanding of the epidemiology and course of pediatric respiratory failure. As part of a clinical trial, we screened all mechanically ventilated children in nine large pediatric intensive care units (ICUs) across North America for 6 consecutive months. Of 6,403 total ICU admissions, 1,096 (17.1%) required mechanical ventilator support for a minimum of 24 hours. Of these, 701 (64%) met one or more exclusion criteria for trial enrollment. Common reasons for exclusion were upper airway obstruction (13.5%) and cyanotic congenital heart disease (11.5%). Life support interventions were restricted for 9.7% of patients, and 5.5% were chronically ventilator dependent. In the patients who were eligible for respiratory failure studies, 62.4% had an acute primary diagnosis of pulmonary disease, 14.2% neurologic disease, and 8.9% cardiac disease. Chronic underlying conditions were present in 43.2% of the patients. The most common acute diagnosis was bronchiolitis in infants (43.6%) and pneumonia in children 1 year old and older (24.5%). Mortality was rare (1.6%), and the median duration of ventilation was 7 days. The design of clinical trials in critically ill children is feasible but must account for the diverse population, infrequent mortality, and short duration of mechanical ventilation.