Few data are available on energy requirements of mechanically ventilated, critically ill children. We measured the resting energy expenditure in 18 mechanically ventilated patients between ages 2 and 18 years, using indirect calorimetry. All patients had fractional inspired oxygen concentration less than 0.6, no spontaneous respirations, hemodynamic stability, and no fever or active infection, and were receiving 5% dextrose. All subjects were hypermetabolic, since the measured resting energy expenditure divided by the predicted basal energy expenditure from the Harris-Benedict equations was 1.48 +/- 0.09 (mean +/- SEM). The energy requirements calculated using "injury factors" and "activity factors" adapted for adults is 1.62 times basal energy expenditure. The injury factor for the pediatric multiple trauma patients should be 1.25 compared with 1.35 in adults. In these pediatric intensive care patients 33% +/- 8% of the energy is derived from carbohydrates, 53% +/- 8% from fat, and 14% +/- 2% from protein oxidation. In individual critically ill pediatric patients, energy requirements should be estimated by measuring their resting energy expenditure whenever possible and adding 5% for their activity. In the absence of the actual measurement of resting energy expenditure, the recommended energy requirement is 1.5 times basal energy expenditure. In this acute phase of injury, the daily nitrogen requirement is 250 mg per kilogram of body weight.