Energy expenditure (EE) of four adult men on a weight-maintenance diet was estimated by use of doubly labeled water, intake balance, and direct and indirect calorimetry. The doubly labeled water (2H218O) method was used to estimate free-living EE for 13 d. Metabolizable energy (ME) intake was used to estimate free-living EE for 1 wk. The subjects' 24-h EE was measured in a dual direct-indirect room calorimeter on 3 alternate days. Estimates of free-living EE as measured by ME intake and doubly labeled water indicate agreement between the two methods (mean difference +/- SEM, -1.04 +/- 0.63%). Measurements of EE with indirect and direct calorimetry are equivalent (mean difference 0.63 +/- 0.44%). The daily EE measured by doubly labeled water in these free-living adults over a 13-d period was 15.01% greater than the 24-h EE measured within the calorimeter.