This is the largest study to estimate prospectively the pediatric population-based acute respiratory distress syndrome incidence and the first incidence study performed during the routine application of lung protective ventilation in children. Our findings support a lower acute respiratory distress syndrome incidence and mortality than those reported for adults. PaO2/FIO2 ratios at acute respiratory distress syndrome onset and at 24 hrs after onset were helpful in defining groups at greater risk of dying (clinical trials registered with http://www.clinicaltrials.gov; NCT 01142544).