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Oxygen cost (OC) is commonly used to assess an athlete’s running economy, although the validity of this measure is often overlooked.This study evaluated the validity of OC as a measure of running economy by comparison with the underlying energy cost (EC). In addition, the most appropriate method of removing the influence of body mass was determined to elucidate a measure of running economy that enables valid interindividual comparisons.One hundred and seventy-two highly trained endurance runners (males, n = 101; females, n = 71) performed a discontinuous submaximal running assessment, consisting of approximately seven 3-min stages (1 km·h−1 increments), to determine the absolute OC (L·km−1) and EC (kcal·km−1) for the four speeds below lactate turn point.Comparisons between models revealed linear ratio scaling to be a more suitable method than power function scaling for removing the influence of body mass for both EC (males, R2 = 0.589 vs 0.588; females, R2 = 0.498 vs 0.482) and OC (males, R2 = 0.657 vs 0.652; females, R2 = 0.532 vs 0.531). There were stepwise increases in EC and RER with increments in running speed (both, P < 0.001). However, no differences were observed for OC across the four monitored speeds (P = 0.54).Although EC increased with running speed, OC was insensitive to changes in running speed and, therefore, does not appear to provide a valid index of the underlying EC of running, likely due to the inability of OC to account for variations in substrate use. Therefore, EC should be used as the primary measure of running economy, and for runners, an appropriate scaling with body mass−1 is recommended.