Objective: Definition of pediatric acute respiratory distress syndrome ( PARDS) is a pragmatic snippet based on the degree of hypoxia at the time of its onset. We aimed to determine whether reclassification by PARDS definition 24 hours after meeting PARDS criteria could have better prognostic ability for risk stratification. Methods: Eight hundred and twenty-six children who admitted to the intensive care unit (ICU) at Severance hospital were screened. Among them, 224 patients diagnosed as PARDS according to the Pediatric Acute Lung Injury Consensus Conference ( PALICC) definition were retrospectively analyzed. Reclassification based on data measured at 24 hours after diagnosis were compared with the initial classification. Primary outcome was in-hospital mortality. Results: Overall mortality rate of PARDS in ICU was 41.9%. Multivariate analysis revealed that the risk for mortality was significantly increased according to severity stratification based on metrics collected at 24 hours after the diagnosis (HR [95%CI], 1.862 [1.005-3.450] for moderate PARDS; 4.181 [2.274-7.686] for severe PARDS). On the other hand, mortality risk increased only for severe PARDS (2.465 [1.488-4.085]) using the initial classification. Oxygenation index (OI) at the time of diagnosis and at 24 hours after onset showed significant associations with mortality (aOR [95%CI], 1.085 [1.043-1.129] for initial OI; 1.241 [1.147-1.342] for OI at 24 hours after diagnosis). Change in OI over the first 24 hours were also significantly associated with mortality (1.050 [1.002-1.101]). Conclusions: Improvements in oxygenation over the first 24 hours were associated with lower mortality. Implementation of reclassification based on oxygenation metrics 24 hours after the initial diagnosis of PARDS seems valid and may contribute to more individualized treatment in PARDS.