In obesity the increased O₂ cost of breathing negatively affects the O₂ cost of exercise and exercise tolerance. The purpose of the study was to determine whether, in obese adolescents, the addition of respiratory muscle endurance training (RMET) (isocapnic hyperpnea) to a standard body mass reduction program decreases the O₂ cost of exercise and perceived exertion and increases exercise tolerance. 9 male obese adolescents (16.0±1.4 years [x±SD], body mass 114.4±22.3 kg) underwent 3 weeks of RMET (5 days/week); 8 age-and sex- matched obese adolescents underwent the standard body mass reduction program (CTRL). Before and after interventions patients performed on a cycle ergometer: incremental exercise; 12-min constant work-rate exercises (CWR) at 65% and 120% of gas exchange threshold (GET) determined before the intervention. Breath-by-breath pulmonary ventilation (V'E) and O₂ uptake (V'O₂), heart rate (HR) and ratings of perceived exertion for dyspnea/respiratory discomfort (RPE_R) and leg effort (RPE_L) were determined. Body mass decreased (by ~3.0 kg) after both RMET and CTRL (P=0.001; GLM for repeated measures). Peak O₂ and peak work rate were not affected by both interventions. During CWR<GET no changes were observed after both interventions. During CWR>GET the O₂ cost of cycling (P=0.014), the slope of V'O₂ vs. time (P=0.012), RPE_R (P=0.012), RPE_L (P=0.016) and HR (P=0.001) decreased following RMET but not following CTRL, whereas V'E did not change. In obese adolescents RMET, superimposed on a standard body mass reduction program, lowered the O₂ cost of cycling and perceived exertion during constant heavy-intensity exercise.