Copyright © 2017 by the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies. Objectives: To derive a relationship between the Spo 2 /Fio 2 ratio and Pao 2 /Fio 2 ratio across the entire range of Spo 2 values (0-100%) and to evaluate whether mortality prediction using the Pediatric Index of Mortality-3 can be improved by the use of Pao 2 /Fio 2 values derived from Spo 2 /Fio 2 . Design: Retrospective analysis of prospectively collected data. Setting: A regional PICU transport service. Patients: Children transported to a PICU. Interventions: None. Measurements and Main Results: The relationship between Spo 2 /Fio 2 and Pao 2 /Fio 2 across the entire range of Spo 2 values was first studied using several mathematical models in a derivation cohort (n = 1,235) and then validated in a separate cohort (n = 306). The best Spo 2 /Fio 2 -Pao 2 /Fio 2 relationship was chosen according to the ability to detect respiratory failure (Pao 2 /Fio 2 ≤ 200). The discrimination of the original Pediatric Index of Mortality-3 score and a derived Pediatric Index of Mortality-3 score (where Spo 2 /Fio 2 -derived Pao 2 /Fio 2 values were used in place of missing Pao 2 /Fio 2 values) were compared in a different cohort (n = 1,205). The best Spo 2 /Fio 2 -Pao 2 /Fio 2 relationship in 1,703 Spo 2 /Fio 2 -to-Pao 2 /Fio 2 data pairs was a linear regression equation of ln[PF] regressed on ln[SF] . This equation identified children with a Pao 2 /Fio 2 less than or equal to 200 with a specificity of 73% and sensitivity of 61% in children with Spo 2 less than 97% (92% and 33%, respectively, when Spo 2 ≥ 97%) in the validation cohort. Pao 2 /Fio 2 derived from Spo 2 /Fio 2 (derived Pao 2 /Fio 2 ) was better at predicting PICU mortality (area under receiver operating characteristic curve, 0.64; 95% CI, 0.55-0.73) compared with the original Pao 2 /Fio 2 (area under receiver operating characteristic curve, 0.54; 95% CI, 0.49-0.59; p = 0.02). However, there was no difference in the original and derived Pediatric Index of Mortality-3 scores and their discriminatory ability for mortality. Conclusions: Spo 2 -based metrics perform no worse than arterial blood gas-based metrics in mortality prediction models. Future Pediatric Index of Mortality score versions may be improved by the inclusion of risk factors based on oxygen saturation values, especially in settings where Pao 2 values are missing in a significant proportion of cases.