SETTING: A network of 22 PICUs in Spain. PATIENTS: Consecutive children (7 d to 15 yr old) with a diagnosis of AHRF, defined by Pao2/Fio2 ratio less than or equal to 300 mm Hg, who needed invasive mechanical ventilation (IMV) using positive end-expiratory pressure (PEEP) greater than or equal to 5 cm H2O and Fio2 greater than or equal to 0.3. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: The primary outcomes were AHRF prevalence and PICU mortality. The secondary outcomes were the prevalence of IMV with PARDS (IMV-PARDS) and the use of adjunctive therapies. There were 6545 PICU admissions: 1374 (21%) underwent IMV and 181 (2.8%) had AHRF. Ninety-one patients (1.4% of PICU admissions, 6.6% of IMV cases, and 50.3% of AHRF cases) met the Second Pediatric Acute Lung Injury Consensus Conference IMV-PARDS criteria. At baseline, mean (± sd) tidal volume was 7.4 ± 1.8 mL/kg ideal body weight, PEEP 8.4 ± 3.1 cm H2O, Fio2 0.68 ± 0.23, and plateau pressure 25.7 ± 6.3 cm H2O. Unlike patients