Accurate determination of the resting energy expenditure (REE) for patients in intensive care units has assumed increasing importance as the adverse effects of underfeeding or overfeeding have become clearer. As a result, indirect calorimetry is frequently used to determine the REE. This technique, however, is neither universally available nor inexpensive. Furthermore, it requires meticulous measurements of gas exchange and technical expertise. Because of this, we have developed a method to determine REE for those patients in intensive care units who have thermodilution pulmonary artery catheters in place. This method requires determining the oxygen consumption from data obtained from such a catheter. The REE is then calculated by multiplying the determined oxygen consumption by the caloric value for oxygen (4.86 kcal/L at a nonprotein respiratory quotient of 0.85). We made 20 simultaneous measurements of REE, comparing our method with indirect calorimetry in 19 patients. There was excellent agreement between both methods (r = 0.90; p less than 0.0001). Therefore, for those patients in intensive care units who require thermodilution pulmonary artery catheters, this method will allow accurate determination of REE and thus caloric requirements, without the need for indirect calorimetry using measurements of gas exchange.