To estimate sources and extent of variation in energy expenditure (EE), as measured by indirect calorimetry in a room-sized human calorimeter, a number of 24-h measurements were compiled. Measured oxygen consumption and carbon dioxide production from alcohol combustion experiments averaged 101.5% of the theoretical value with a coefficient of variation (CV) of 1.4%. Experiment 1 consisted of four men who had the following averages: age, 41 y; height, 179 cm; weight, 84.6 kg; and fat, 23.5%. Five measurements, separated by 1 d, were made on each subject. Daily and basal EE averaged 2852 and 1691 kcal/d, respectively, with a within-subject CV of 2.7% and 2.4%, respectively. Experiment 2 consisted of five men who had the following averages: age, 48 y; height, 181.6 cm; weight, 87 kg; and fat, 23%. Five measurements made on each subject were separated by 1-3 wk. Daily and basal EE averaged 2619 and 1837 kcal/d, respectively, with a within-subject CV of 4.6% and 2.9%, respectively.