Objective: To examine fat oxidation rates during exercise in lean and obese pubescent children. Design: A graded leg cycle ergometry test was performed by two groups of pubescent boys (13 lean: mean (SD) age 12.0 (0.5) years, body mass index (BMI) 18.56 (1.12) kg/m2; 17 obese: mean (SD) age 12.1 (0.1) years, BMI 26.68 (3.37) kg/m2; p<0.001). The first step of the test was fixed at 30 W and power was gradually increased by 20 W every 3.5 min. The mean ventilatory gas measurement was obtained during the last 30 s of each step for calculation of fat oxidation rate vs exercise intensity. Results: At low intensity (0–30% of peak oxygen consumption) when fat-free mass is considered, the fat oxidation rate was identical for the two groups. At higher intensities (40%, 50% and 60% of peak oxygen consumption) the fat oxidation rate was significantly higher in lean boys than in obese boys. Conclusion: These results confirm that obese pubertal boys have fat-free mass decreased capacities to use fat during moderate exercise. The findings suggest that obese boys need to practise physical activity at a lower intensity than healthy boys to enhance lipolysis and diminish adipose tissue and the consequences of obesity.