Fifteen obese children, aged 4 to 16 years and ranging in body weight from 27.3 to 95 kg (median 67.5) and percentage overweight from 22 to 127% (median 40), underwent graded exercise testing on a treadmill and were compared with healthy peers of comparable age. Cardiorespiratory performance capacity was assessed by determination of the ventilatory threshold. This was defined as the highest oxygen uptake at which the pulmonary ventilation stops to increase linearly with increasing exercise intensity. The ventilatory threshold, expressed as ml O2/min/kg, was significantly (p less than 0.05) lower than in normal children and averaged 70.6 +/- 13.5% of the normal mean value, matched for age. The habitual level of physical activity, assessed by a questionnaire, was 27% lower (p less than 0.01) in the obese children compared to healthy controls. As the ventilatory threshold was strikingly lower and also exceeded sooner in the majority of the obese children, compared with normal controls, it may be assumed that obese children avoid moderate or strenuous exercise, because of the higher degree of effort needed. This may contribute to the maintenance of overweight.