The thermic effect of a meal (TEM) was measured in a group of 10 prepubertal obese children before (OB) and after (OA) weight reduction, and in a group of 10 age-matched control children (C) of normal body weight. Following a hypocaloric balanced diet for 6+/-1 months, the obese children lost 5.2+/-1.3 kg ie 11% of their initial body weight. The thermic response to the mixed liquid meal-fed at an energy level corresponding to 30% of the 24 h premeal resting metabolic rate-was found to be significantly lower in OB than in C children (61+/-25 kJ. 3h-1 vs 79+/-21 kJ. 3h-1, P less than 0.05), despite their higher test meal energy. After slimming, the TEM of obese children increased towards the controls' values (73+/-30 kJ. 3h-1). These results support the hypothesis of the existence of a moderate thermogenic defect in some obese children which represents a consequence rather than an aetiological factor of obesity.