Background Acute hypoxemic respiratory failure (AHRF) often develops acute respiratory distress syndrome (ARDS),and its incidence and mortalities in critically ill pediatric patients in China were 2％ and 40％ respectively.This study aimed at prospectively investigating incidence,causes,mortality and its risk factors,and any relationship to initial tidal volume (VT) levels of mechanical ventilation,in children ≤5 years of age with AHRF and ARDS.Methods In 12 consecutive months in 23 pediatric intensive care units (PICU),AHRF and ARDS were identified in those requiring ＞12 hour intratracheal mechanical ventilation and followed up for 90 days or until death or discharge.ARDS was diagnosed according to the American-European Consensus definitions.The mortality and ventilation free days (VFD) were measured as the primary outcome,and major complications,initial disease severity,and burden were measured as the secondary outcome.Results In 13 491 PICU admissions,there were 439 AHRE,of which 345 (78.6％) developed ARDS,resulting in incidences of 3.3％ and 2.6％,and corresponding mortalities of 30.3％ and 32.8％ respectively along with 8.2 and 6.7 times of relative risk of death in those with pneumonia (62.9％) and sepsis (33.7％) as major underlying diseases respectively.No association was found in VT levels during the first 7 days with mortality,nor for VT at levels ＜6,6-8,8-10,and ＞10 ml/kg in the first 3 days with mortality or length of VFD.By binary Logistic regression analyses,higher pediatric risk of mortality score Ⅲ,higher initial oxygenation index,and age ＜1 year were associated with higher mortality or shorter VFD in AHRF.Conclusions The incidence and mortalities of AHRF and ARDS in children ≤5 years were similar to or lower than the previously reported rates (in age up to 15 years),associated with initial disease severity and other confounders,but causal relationship for the initial VT levels as the independent factor to the major outcome was not found.