Background: Optimal energy provision, guided by measured resting energy expenditure (REE), is fundamental in the care of critically ill children. REE should be determined by indirect calorimetry (IC), which has limited availability. Recently, a novel equation was developed for estimating REE derived from carbon dioxide production (VCO2). The aim of this study was to validate the accuracy of this equation in a population of critically ill children following cardiopulmonary bypass (CPB). Methods: This is an ancillary study to a larger trial of children undergoing CPB. Respiratory mass spectrometry was used measure oxygen consumption (VO2) and VCO2. REE was then calculated according to the established Weir equation (REEW) and the modified, VCO2-based equation (REECO2). The agreement between the 2 measurements was assessed using Bland-Altman plots and mixed-model regressions accounting for repeated measures. Results: Data from 104 patients, which included 575 paired measurements, were included. The agreement between REEW and REECO2 was biased during the 72-hour observation period post CPB, with a mean percentage error between measurements of 11% (±7%). The most important determinant of the bias with the VCO2-based equation was the respiratory quotient (RQ). The percentage error between REEW and REECO2 dropped to 4.4% (±2.4%) in those with an RQ between 0.8 and 1. The within-subject variability for RQ in this cohort was wide (11%). Conclusions: IC remains the most accurate method to determine the REE of critically ill patients. Widespread availability of VCO2 data renders VCO2-based approaches to measurement of REE attractive; however, further research is needed to ensure that REE is estimated accurately.