Resting energy expenditure (REE) is the largest component of total daily energy expenditure. Objectives of this study were to examine whether differences in REE exist after obesity develops in a group of children and adolescents, and to determine the effects of body composition, gender, age, pubertal development and parental obesity on REE. In 116 Caucasian children and adolescents (57 obese and 59 non-obese), aged 7.8 to 16.6 years, REE was assessed by open-circuit indirect calorimetry and different anthropometric variables and bioelectrical impedance were obtained (weight, height, skinfold thicknesses, waist and hip circumferences). Anthropometric indices and body compartments were calculated: the body mass index, surface area (SA), fat-free mass (FFM), fat-mass (FM) and percentage of FM. Differences between obese and non-obese subjects were tested and stepwise multiple regression analysis was performed with REE as dependent variable. Results show that REE was significantly higher in obese than in non-obese children and adolescents but REE/FFM ratio was not significantly different between these groups. In the non-obese group, FFM explained 73.1% of the variability in REE and gender, age and SA added 3.8%, 2.6%, and 2.6% to it, respectively. In the obese group, FFM was also the most powerful predictor of REE with 72.3%, followed by waist circumference and age with 2.5% and 2.1%, respectively. These results show that REE differences between obese and lean children do not seem to justify the maintenance of obesity. The main determinant of REE is FFM in both groups. No significant contribution of FM, pubertal development or parental obesity in REE was found in children and adolescents.