The resting metabolic rate (RMR) was measured in obese (n. 14) and normo-weight (n. 14) pre-pubertal children using indirect calorimetry: correlations between RMR and anthropometric and metabolic parameters were also assessed in order to identify the existence of a causal relationship between the reduction of the RMR and the onset of obesity in children. RMR was expressed as an absolute value and appeared to be higher in obese subjects in comparison to controls (5138 +/- 498 kJ/d vs 4443 +/- 481, p less than 0.0001). If corrected for Kg of fat free mass (FFM), the anthropometric variable which accounts for the major (75%) variations of RMR, resting metabolism in obese subjects was lower than that in normal weight controls (173.2 +/- 20.1 kJ/kg FFM/d vs 189.9 +/- 17.6, p less than 0.05). The lower percentage of metabolic active organs which make up the lean body mass in FFM (brain, heart, liver and kidneys) and the absence of a defect in resting thermogenesis in the obese child could nevertheless explain the reduction of RMR/kg FFM observed in the study.