Objective:To determine whether the energy expenditure of mechanically ventilated multiple trauma patients correlates with the severity of injury and illness indices before important systemic infection has complicated the clinical course, and to compare the energy expenditure with the energy expenditure expected from the Harris-Benedict equation adjusted with correction factors for trauma.Design:Prospective, clinical study.Setting:General intensive care unit of a university teaching hospital.Patients:Immediate multiple trauma adult patients who required mechanical ventilation.Interventions:Metabolic cart connected to the ventilator.Measurements and Main Results:Data on admission to the emergency department and during the first 24 hrs of intensive care unit admission were collected for computation of severity of injury and illness indices, respectively. Resting and total energy expenditures were derived at least 48 hrs after intensive care unit admission by continuous indirect calorimetry. Predicted basal energy expenditure was obtained using the Harris-Benedict equation and predicted total energy expenditure was calculated using the Harris-Benedict value adjusted with correction factors for trauma. Twenty-six multiple trauma adult patients completed the study. No statistically significant correlations were observed between both the resting energy expenditure and the total energy expenditure and the Injury Severity Score, Revised Trauma Score, Simplified Acute Physiologic Score II, Acute Physiology and Chronic Health Evaluation II score, and Glasgow Coma Scale score. A regression model of total energy expenditure was developed with the following variables: Harris-Benedict equation, heart rate, and minute ventilation (p= .01; r2= .74). The resting energy expenditure/predicted basal energy expenditure ratio was 1.17 ± 0.2 and the total energy expenditure/predicted total energy expenditure ratio was 0.76 ± 0.1.Conclusions:In mechanically ventilated multiple trauma patients the energy expenditure is not correlated to the severity of injury and illness indices but is dependent on the Harris-Benedict equation in addition to heart rate and minute ventilation. Furthermore, this patient population is characterized by a moderate state of hypermetabolism, and the Harris-Benedict prediction modified with correction factors for trauma systematically overestimates the total energy expenditure.