Resting energy expenditure (EE) is often used as the basis of nutritional support for critically ill patients but whether resting EE is representative of total daily EE is not known. EE was measured for 24 h in ten mechanically ventilated, critcally ill patients (average Acute Physiology and Chronic Health Evaluation II score 23) to determine EE, resting EE, and the energy expended during various ICU activities. Although activities, such as weighing the patient on a sling-type bed scale, repositioning, and chest physiotherapy resulted in dramatic EE increases above resting levels (36%, 31%, and 20%, respectively), the actual contribution of these activities to total EE was small (1.1%, 2.1%, and 3.6%, respectively). The mean measured resting EE was 47.3 ± 22.3% above mean predicted EE based on the Harris and Benedict equation, and the mean total 24-h EE was 6.9 ± 2.6(SD)% above the mean measured resting EE. In this group of mechanically ventilated, critically ill patients, an activity factor of no greater than 10% above resting EE is appropriate.