Direct measurements of Resting Metabolic Rate (RMR) are important in the nutrition support of critically ill patients. Indirect Calorhnetry (1C) is considered the gold standard method to assess RMR in these patients. Circulatory Indirect Calorhnetry (Pick Method) using a thermodilution technique with room-temperature injectate has been proposed as an alternative method to evaluate energy expenditure for those patients who have thennodihition pulmonary artery catheters in place. We compared these two methods in six MICU patients and studied the metabolic and hemodynamic predictors of oxygen consumption (VÜ2). VOi measured by the Pick method (VO2-Fick) correlated well with VO2 measured by 1C (VO2-gas) (r=0.96, p<0.0001). The limits of agreement were -42 and 32.4 ml/min for the VO2-Fick according to the Bland- Altman method. No correlation was found between VO2-gas and clinical variables (weight, height, body temperature, hemoglobin, Glasgow score, Apache II score) using stepwise multiple regression analysis. The respiratory quotients ranged from 0.76 to 0.95. We conclude that the Fick method using room temperature injectate can precisely estimate RMR in critically ill patients in a MICU.