The Pediatric Index of Mortality 3 (PIM3) is a scale that estimates the risk of mortality in children admitted to the Pediatric Intensive Care Unit (PICU) within the first hour of admission. to validate the PIM3 scale in pediatric population admitted to PICU at altitudes over 2,500 meters above sea level (m.a.s.l.), and to evaluate whether lactate and the oxygen saturation/fraction of inspired oxygen (SpO2/FiO2) index improve its ability to predict mortality. A prospective multicenter study was carried out in 10 PICUs in Colombia between November 2016 and June 2017. Variables were collected for the calculation of PIM3, serum lactate, and SpO2/FiO2 index. Their discrimination capacity was validated with the area under the ROC curve and calibration using the Hosmer Lemeshow goodness-of-fit test. 2,803 admissions were included, with an overall mortality of 4.8% and a standardized mortality ratio of 1.1 (95%CI 1.00-1.3). PIM3 showed low calibration (p < 0.001) and adequate discrimination for the studied population (ROC 0.84; 95%CI 0.83-0.86). The addition of serum lactate and SpO2/FiO2 allowed a good calibration (p = 0.75) and maintained a high discrimination of mortality (ROC 0.86; 95%CI 0.84-0.87). The performance of the PIM3 scale has adequate discrimination, but low calibration in children admitted to PICU at above 2500 m. a. s. l. Lactate and SpO2/FiO2 variables improved its predictive ability.