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Vigorous aerobic exercise may improve aerobic capacity (VO 2max) and cardiometabolic profiles in adolescents with obesity, independent of changes to weight. Our aim was to assess changes in estimated VO 2max in obese adolescents following a 6-week exercise program of varying intensities. Adolescents with obesity were recruited from an American mid-west children’s hospital and randomized into moderate exercise (MOD) or high intensity interval exercise (HIIE) groups for a 6-week exercise intervention, consisting of cycle ergometry for 40 minutes, 3 days per week. Heart rate was measured every two minutes during each exercise session. Estimated VO 2max measured via Åstrand cycle test, body composition, and physical activity (PA) enjoyment evaluated via questionnaire were assessed pre/post-intervention. Twenty-seven adolescents (age 14.7±1.5; 17 female, 10 male) completed the intervention. Estimated VO 2max increased only in the HIIE group (20.0±5.7 to 22.7±6.5 ml/kg/min, p= 0.015). The HIIE group also demonstrated increased PA enjoyment, which was correlated with average heart rate achieved during the intervention (r= 0.55; p= 0.043). Six weeks of HIIE elicited improvements to estimated VO 2max in adolescents with obesity. Furthermore, those exercising at higher heart rates demonstrated greater PA enjoyment, implicating enjoyment as an important determinant of VO 2max, specifically following higher intensity activities.