Exercise capacity can be predicted by a 2-factor model comprising lung function and leg muscle function (Lands et al, Clin Sci (Lond), 83: 391-7, 1992). Adolescents living with obesity (AlwO) can have limited exercise capacity, but the reasons are unclear. No study has looked at leg muscle function during cycling and its contribution to cycling exercise capacity in AlwO. 8 nonobese (3M/5F) and 7 (3M/4F) obese (BMI>95percentile) adolescents were studied. Leg muscle mass was measured by DEXA, lung function (FEV1 absolute and %predicted) by spirometry, leg muscle function by 30-sec isokinetic cycle work capacity, and maximal exercise capacity (VO2peak absolute and %predicted) by progressive cycle ergometry. Nonobese and obese groups did not differ in age (17.3±2.0 vs 16.3±1.9 yr), but the nonobese group were shorter (158.7±8.4 vs 170.8±6.3 cm), weighed less (57.6±9.1 vs 111.1±21.4 kg), had less leg muscle mass (13.5±3.1 vs 20.8±2.9 kg), and a lower BMI (22.8±3.3 vs 37.9±5.6 kg/m2). The groups did not differ in FEV1 absolute (3.3±0.8 vs 4.1±0.9 L (p=0.07)) or %predicted (101.2±12.9 vs 111.9±10.1 %), 30-second leg work capacity (8.0±3.0 vs 9.3±3.4 kJ), or VO2peak absolute (1.83±0.56 vs 2.24±0.55 L/min) or %predicted (84.8±17.1 vs 73.7±23.9 %). 30-second leg work capacity related to leg muscle mass (r=0.63, p<0.05), with no significant group effect. Absolute VO2peak was related to absolute FEV1 (r=0.88, p<0.001) and 30-second leg work capacity (r=0.83, p<0.001) alone, and in combination (r=0.91, p<0.001), with no significant group effect.Conclusion: Exercise capacity was preserved in AlwO. Exercise capacity was dependent upon lung function and leg muscle work capacity, which was related to leg muscle mass.