The purpose of this study was to assess the validity and reliability of the MedGem[TM] device to measure resting metabolic rate (RMR) in children. Sub-jects included 59 children (29 boys, 30 girls; mean age, 11.0 +/- 0.2 y). Subjects were given 4 RMR tests during 1 test session, cconsisting of 2 Douglas bag and 2 MedGem tests, in random counterbalanced order. No significant differences were found between Douglas bag and MedGem systems for oxygen consump-tion (209 +/- 5 and 213 +/- 5 mL/min, respectively, P = 0.106, r = 0.911, mean +/- standard deviation absolute difference 3.72 +/- 17.40 mL/min) or RMR (1460 +/- 39 and 1477 +/- 35 kcal/d, P = 0.286, r = 0.909, mean +/- standard deviation absolute difference 17.4 +/- 124 kcal/d). Standard error of estimates for oxygen consumption and RMR were 17.4 mL/min and 124 kcal/d, respectively. In conclusion, these data indicate that the MedGem is a reliable and valid system for measuring oxygen consumption and RMR in children.