To investigate sleeping energy expenditure in paediatric patients with obstructive sleep apnoea syndrome. Retrospective study. Setting. University teaching hospital, Hong Kong. A retrospective analysis comparing sleeping energy expenditure in patients with confirmed obstructive sleep apnoea syndrome and control subjects matched for age, sex, and ideal weight for body height. Subjects were recruited from the Paediatric Chest Clinic and all had undergone overnight polysomnography and sleeping energy expenditure measurement by open-circuit indirect calorimetry using a metabolic cart and canopy system. The measurements were taken during slow-wave sleep. Twenty-four cases with obstructive sleep apnoea syndrome (apnoea hypopnoea index >5) and 23 control subjects were studied. Mean age and ideal weight for body height were 9.4 (standard deviation, 3.9) years and 152.5% (27.2%), respectively. Mean sleeping energy expenditure corrected for body weight for the cases and the control group were 44.83 (standard deviation, 10.49) Kcal/day and 40.71 (10.60) Kcal/day, respectively. Sleeping energy expenditure was not found to be associated with the severity of obstructive sleep apnoea syndrome (r=0.34, P=0.27). A trend towards greater sleeping energy expenditure was found in patients with obstructive sleep apnoea syndrome. Further studies on the metabolic aspects of this condition are required.