To apply and determine whether standardized mortality scores are appropriate to predict the risk of mortality in mechanically ventilated pediatric patients, 150 patients were retrospectively evaluated. Pediatric risk of mortality (PRISM) III-24 and pediatric index of mortality (PIM)-2 scores were unable to discriminate survivors and nonsurvivors; the observed mortality rate was lower than expected mortality rates. Oxygenation index (OI) was calculated at 0, 12, 24, and 72 hours of ventilation. OI-12 and OI-72 were found to be higher in nonsurvivors. PRISM III-24 and PIM-2 scores failed to predict mortality risk in mechanically ventilated pediatric patients. OI can be used to predict degree of respiratory failure and mortality risk.