OBJECTIVE: To assess the validity and reliability of a hand-held indirect calorimeter. DESIGN: Resting metabolic rate (RMR) was measured on two separate mornings. SUBJECTS: A heterogeneous sample of 41 healthy adults. MEASUREMENTS: RMR using both a metabolic cart (Sensormedics 2900, SM-2900) and a hand-held indirect calorimeter (BodyGem™, BG). RESULTS: There were no trial-to-trial differences in RMR measured by the BG (6756 ± 163 vs 6697 ± 163 kj/day) or the SM-2900 (6400 ± 163 vs 6396 ± 167 kj/day). RMR measured by the BG was significantly higher than that measured by the SM-2900 during both trials. In a sample of 10 subjects, the energy cost of holding the BG in position was determined to be (0.17 ± 0.04 kj/ min, or 255 ± 84 kj/day). After applying this adjustment, the differences between systems were no longer significant during trial 1 (mean difference = 101 ± 67kj/day) or trial 2 (46 ± 75kj/day). In overweight and obese individuals, RMR measured by the BodyGem™ was more accurate than that estimated by the Harris-Benedict equations. CONCLUSION: The BodyGem™ provides valid and reliable measurements of RMR. The BodyGem™ produces significantly higher values than the Sensor Medics 2900 indirect calorimeter, with the increase largely due to an increased energy demand required to hold the BG in position.