The purpose of this study was to determine the association of injury type (trauma, surgery, medical disease), systemic inflammatory response syndrome (SIRS) and fever with the degree of hypermetabolism in critically ill patients. Medical records of 204 critically ill, mechanically ventilated injured, surgical and medical patients were reviewed for indirect calorimetry and associated data. Analysis of variance and covariance was used to test the effects of injury, fever and SIRS on the degree of hypermetabolism. All injury types were found to be hypermetabolic. Analysis of variance of hypermetabolism with injury type and presence of fever as main effects revealed a significant increase in hypermetabolic response from fever, of similar magnitude across all injury types. Subjects with SIRS were significantly more hypermetabolic than subjects without SIRS. However, analysis of variance indicated no effect for SIRS but a significant effect for fever in increasing the hypermetabolic response. It is concluded that fever portends a magnification of the hypermetabolic response, being similar across injury types. SIRS does not identify hypermetabolic patients independent of fever. The host response to injury, not the injury itself, determines metabolic rate in critically ill patients. Neither SIRS nor injury type should be used to classify hypermetabolic states without stratifying for presence of fever.