Aims & Objectives: Hypoxemia assessment commonly involves the PaO2/FiO2 ratio and recently the SpO2/FiO2 ratio. However, the effect of altitude on these indices remains uncertain. We aimed to evaluate altitude's impact on the PaO2/FiO2 and SpO2/FiO2 correlation in pediatric acute respiratory failure (ARF) and analyze its association with outcomes in Latin American centers at varying altitudes. Methods: We conducted a secondary analysis of cross-sectional data from the LARed Network pediatric ARF cohort between April 2017 and April 2023. Patients under 18 years admitted to pediatric intensive care units (PICUs) for ARF, with recorded SpO2/FiO2 on admission, were included. Cases were categorized by altitude above or below 1500 m.a.s.l. Linear regression compared PaO2/FiO2 and SpO2/FiO2, adjusting for altitude, pH, and CO2 pressure. Mortality and PICU length of stay were analyzed using multiple logistic regression and Cox regression, considering altitude, SpO2/FiO2, and relevant factors. Results: Out of 9,206 admissions, 4,194 (46%) met criteria, with 831 (9%) having PaO2 measurements. Linear regression revealed a strong relationship between PaO2/FiO2 and SpO2/FiO2 (R2=0.43). Cox regression identified associations with PICU length of stay for (SpO2/FiO2)/100 (HR 0.97, 95%CI 0.94-0.99), altitude > 1500 m.a.s.l. (HR 1.18, 95% CI 1.04-1.34), and their interaction (HR 0.91, 95% CI 0.85-0.96). However, logistic regression showed no significant associations between mortality and (SpO2/FiO2)/100 (OR 0.92, 95%CI 0.72-1.19), Altitude >1500 m.a.s.l. (OR 1.99, 95%CI 0.81-4.86), or their interaction (SpO2/FiO2)/100 x Altitude >1500 m (OR 1.07, 95%CI 0.67-1.71). Conclusions: Altitude marginally affects the PaO2/FiO2 relation to SpO2/FiO2 and admission SpO2/FiO2 is associated with length of stay, but not mortality. Keywords: Altitude, Oxygen Ratios, Latin america, acute respiratory failure, Mortality