Objective:To determine whether oxygen consumption (&OV0312;O2), CO2production, and resting energy expenditure (REE) in critically ill patients differ in varying grades of systemic inflammatory response syndrome (SIRS).Design:Prospective, clinical study.Setting:Intensive care unit at a university hospital.Patients:Twenty-six critically ill patients requiring mechanical ventilation.Interventions:None.Measurements and Main Results:A total of 100 metabolic measurements were performed. The grade of SIRS and the Acute Physiology and Chronic Health Evaluation II score were evaluated at the time of the metabolic cart study. &OV0312;O2and REE differed among the groups inadequate for SIRS (non-SIRS), with SIRS without infection (nonseptic SIRS), and with SIRS with infection (septic SIRS) (125 ± 37 mL/min/m2and 855 ± 204 kcal/day/m2, 135 ± 33 mL/min/m2and 948 ± 214 kcal/day/m2, and 166 ± 55 mL/min/m2and 1149 ± 339 kcal/day/m2, respectively;p< .005). Patients with septic SIRS had higher &OV0312;O2and REE than patients with non-SIRS and nonseptic SIRS.Conclusion:&OV0312;O2and REE differ among groups of patients with non-SIRS, nonseptic SIRS, and septic SIRS. Patients with septic SIRS have higher &OV0312;O2and REE than patients with non-SIRS or nonseptic SIRS. The present study shows that classifying patients into three grades (non-SIRS, nonseptic SIRS, and septic SIRS) is a valid predictor of metabolic stress in critically ill patients.