To determine the effect of adiposity on adolescent energy expenditure compartments, basal energy expenditure (BEE) together with the thermic effect of food at rest and during post-exercise recovery were compared in eight lean (BMI < 25 kg/m2) and eight obese (BMI > 27 kg/m2) 15-year-old adolescent males at the same stage of pubertal development. Both groups were of equivalent fat-free mass. Continuous energy expenditure was measured by open circuit indirect calorimetry for periods of up to three hours following: (i) an overnight fast; (ii) a test meal; and (iii) moderate exercise after a further serving of the test meal. Repeat baseline measurements were taken on the following day. Absolute basal energy expenditure was higher in obese than in lean subjects. No significant differences were observed between groups in relation to BEE per kg total fat free-mass. Thermogenesis was significantly greater in the lean relative to the obese group under resting conditions (61.1 +/- 8.9 vs. 41.4 +/- 5.1 kcal/3h; P < 0.05) and in the post-exercise period (69.4 +/- 6.3 vs. 49.0 +/- 5.6 kcal/3h; P < 0.05). Of the body composition parameters, percentage fat mass was the best predictor of the thermic effect of food at rest (R = -0.53; P = 0.03) and post-exercise recovery (R = -0.61; P = 0.012). The results indicated that: (i) even when lean and obese adolescents are comparable with respect to fat-free mass, thermogenesis is blunted in obese subjects; and (ii) the best body composition predictor of thermogenesis in adolescents is percentage fat mass.