The purpose of this study was to quantify the changes in energy expenditure and protein turnover imposed by blunt trauma in children and to correlate them with the Injury Severity Score (ISS). We studied 19 children (mean age 10 +/- 1 year, mean ISS 20 +/- 2). Basal metabolic rate (BMR) was measured in the postabsorptive state by open-circuit indirect calorimetry. Whole body protein turnover (Q) and synthesis (S) were determined by the 15N enrichment of urinary ammonia in a 12-hour collection following a single dose of 15N glycine. Twelve-hour total urinary nitrogen excretion (E) was also determined. Because nitrogen intake was 0 during the study period, Q was equivalent to protein breakdown (B). Eleven patients were restudied at 3- to 5-day intervals during hospitalization and eight were restudied after discharge (mean 34 +/- 6 days post injury). There was a significant increase in BMR, Q, S, and E following injury, when compared with post injury baseline values. However, while BMR increased by 14%, there were 93% and 82% increases in Q (B) and S, respectively. Negative nitrogen balance resulted from the fact that protein breakdown increased more than protein synthesis. The initial increase in BMR varied directly with the severity of injury, as reflected in the ISS (r = 0.56, P less than .02). There was no significant correlation between ISS and any of the parameters of protein metabolism. These results suggest that the metabolic response of pediatric patients to multiple trauma may differ from that of adults. In addition, they imply that the ISS may not be a reliable indicator of the severity of tissue injury.