The increased energy expenditure associated with severe trauma to the head appears genuine but exhibits wide variation in its magnitude. Patients with severe acute trauma to the head without barbiturate treatment [sodium pentobarbitol] are hypermetabolic with an average energy expenditure 26% over predicted. Barbiturate therapy abolishes this hypermetabolism and decreases energy expenditure to 14% below predicted. In the individual patient, there appears to be a close relationship between the degree of suppression of energy expenditure and the serum barbiturate level. This relationship would appear to be different in each patient, and therefore, for this group, a significant correlation between energy expenditure and serum barbiturate level does not exist. The wide variability of energy expenditure in individual patients makes the estimation of energy expenditure by population predictive formulas imprecise. This may lead to incorrect estimates of caloric requirements and inappropriate provision of exogenous energy substrates. Although for those patients receiving barbiturate therapy, the relationship between energy expenditure and serum barbiturate levels in the individual may further aid in estimating the caloric expenditure for each individual, to provide appropriate amounts of calories to the patient with trauma to the head, energy expenditure should be measured in each instance.