Cardiorespiratory fitness (CRF) is a independent risk factor for CVD and all-cause mortality. Maximal oxygen uptake (VO2max), is considered the gold standard measurement of CRF. Due to its effort dependency, a true plateau in VO2 during incremental exercise is often not attained, particularly in overweight and obese pediatric populations 1. The oxygen uptake efficiency slope (OUES) has been proposed as an objective and effort independent submaximal measure of cardiopulmonary reserve. Studies in healthy and obese children have reported a strong positive relation between OUES and VO2max. Children with high CRF have higher OUES values than those with low CRF 2. Furthermore, improvements in CRF following exercise training, are associated with restoration of endothelial function (EF) in obese children 3. The purpose of this study was to examine the relation between OUES, VO2max and EF in healthy male adolescents. ado