['In critically ill patients, energy requirements are frequently calculated as a multiple of total body weight presuming a linear relationship between total body weight and resting energy expenditure (REE); however, it is doubtful if this estimation of energy needs should be applied to all patients, particularly to overweight patients, since adipose tissue has a low contribution to REE. This study was undertaken to test the hypothesis that REE adjusted for total body weight decreases with increasing body mass index in critically ill patients. Additionally, measured REE was compared with three predictive equations.', 'Clinical study in a university hospital intensive care unit.', 'One hundred critically ill patients admitted to the intensive care unit.', 'Patients were included into four groups according to their body mass index (normal weight, pre-obese, obese, and morbidly obese). Measured REE was assessed using indirect calorimetry. Energy needs were calculated using the basal metabolic rate, the Consensus Statement of the American College of Chest Physicians (REEacs), and 25[Symbol: see text]kcal/kg of ideal body weight (REEibw). Adjusted REE was 24.8 +/- 5.5 kcal/kg in normal weight, 22.0 +/- 3.7 kcal/kg in pre-obese, 20.4 +/- 2.6 kcal/kg in obese, and 16.3 +/- 2.3 kcal/kg in morbidly obese patients (p < 0.01). Basal metabolic rate underestimated measured REE in normal weight and pre-obese patients. REEacs and REEibw over- and underestimated measured REE in overweight patients, respectively.'] Is weight-adjusted resting energy expenditure constant in critically ill patients?