We measured body composition and energy expenditure during walking and running on a treadmill in 40 prepubertal children: 23 obese children (9.3 +/- 1.1 years of age; 46 +/- 10 kg (mean +/- SD)) and 17 nonobese matched control children (9.2 +/- 0.6 years of age; 30 +/- 5 kg). Energy expenditure was assessed by indirect calorimetry with a standard open-circuit method. At the same speed of exercise, the energy expenditure was significantly (p < 0.01) greater in obese than in control children, in both boys and girls. Expressed per kilogram of body weight or per kilogram of fat-free mass, the energy expenditure was comparable in the two groups. Obese children had a significantly (p < 0.01) larger pulmonary ventilatory response to exercise than did control children. Heart rate was comparable in boys and girls combined but significantly higher (p < 0.05) in obese subjects, if boys and girls were analyzed separately. These data indicate that walking and running are energetically more expensive for obese children than for children of normal body weight. The knowledge of these energy costs could be useful in devising a physical activity program to be used in the treatment of obese children.