Study 1: A high cardiorespiratory fitness (CRF) level is positively associated with a more favorable cardiometabolic health profile in adolescents. Limited information exists on the relation between CRF and vascular health in healthy adolescents. The purpose of this study was to compare cardiovascular disease (CVD) risk factors and vascular health in healthy low fit (LF), moderate fit (MF) and high fit (HF) male adolescents. LF male adolescents had significantly poorer endothelial dependent dilation (EDD) and cardiovascular health profile and significantly higher carotid intima media thickness (cIMT) compared to both MF and LF. There was a significant positive relation between V̇O2max and EDD and a significant inverse association between V̇O2max and cIMT in healthy male adolescents. Study 2: The oxygen uptake efficiency slope (OUES) has been proposed as an objective and effort independent submaximal measure of CRF and may serve as an alternative to V̇O2max when assessing CRF in adolescents. This study evaluated OUES and vascular health in LF, MF and HF healthy male adolescents with low, moderate and high CRF. Maximal and submaximal OUES expressed relative to body weight were significantly higher in HF than both MF and LF. There was a significant positive relation between both maximal and submaximal OUES and EDD and a significant inverse relation between maximal and submaximal OUES and cIMT. In a stepwise multiple linear regression analysis that included V̇O2max, maximal OUES and submaximal OUES, only V̇O2max was a significant predictor of EDD. Study 3: This study compared physical activity (PA) levels, sedentary behaviour, CVD risk factors and vascular health in LF, MF and HF apparently healthy male adolescents. Time spent in moderate-to-vigorous PA (MVPA) was significantly higher and time spent sitting/lying was significantly lower in HF than both MF and LF. Time spent in light intensity PA (LIPA) was significantly higher in HF than LF. Time in MVPA and LIPA were both significantly and positively related with EDD and inversely related to cIMT. There was a significant inverse relation between time sitting/lying and EDD and a significant positive relation between time sitting/lying and cIMT. Conclusion: cIMT was significantly higher and EDD was significantly lower in LF than both MF and HF. While OUES, PA and sedentary time were all significantly related to markers of vascular health, a more robust relation was observed between V̇O2max and vascular health. Furthermore, V̇O2max was independently related to EDD and cIMT.