PURPOSE: This research aims to observe sex difference in oxygen consumption (VO2) expressed in absolute terms and relative to body weight and to fat-free mass in adolescent children while exercising at submaximal and maximal intensity. METHODS: Body composition (Bod Pod, COSMED) and cardiorespiratory fitness (K5, COSMED) were assessed on twenty-two recreationally active, healthy adolescents (age 16-17 years; 9 males, fat mas% 16.8 ± 8.0; 13 females, fat mas% 27.4 ± 5.4). A 15 watts/min incremental test to exhaustion was performed on a cycle ergometer to measure VO2 at the aerobic threshold (AerT) and at peak exercise (VO2peak) using breath-by-breath gas analysis. Values were expressed in both absolute terms and normalized by body mass and fat-free mass. The AerT was identified using the V-slope technique and as the lowest respiratory equivalent for oxygen. RESULTS: The VO2 at the AerT was higher in males than females when expressed in absolute terms (1184 ± 222 vs 942 ± 202 ml/min, p = 0.015) and normalized by body weight (16.9 ± 2.2 vs 16.4 ± 3.1 ml/kg/min) but the trend was inverted when VO2 was normalized by FFM (20.43 ± 2.92 vs 21.57 ± 2.86 ml/kg/min). The same trend was observed at peak exercise. Males reported higher VO2peak than females when expressed in absolute terms (1792 ± 250 ml/min vs 2344 ± 554 ml/min, p < 0.01) and normalized by body weight (33.5 ± 6.1 vs 30.6 ± 5.5 ml/kg/min) but lower when normalized by FFM (40.1 ± 5.7 vs 41.4 ± 8.0 ml/kg/min). Male adolescents exerted higher power than females both at the AerT (63 ± 13 vs 57 ± 20 watts) and peak effort (198 ± 40 vs 160 ± 20 watts, P < 0.01). CONCLUSIONS: This study provides reflection on whether VO2 should be normalized by total mass or FFM in adolescent males and females.