The purpose of this study was to assess the validity and reliability of the FitMate metabolic system (Cosmed, Rome, Italy) in measuring oxygen consumption and estimating resting metabolic rate (RMR). The FitMate is a new, small (20 x 24 cm) metabolic analyzer designed for measurement of oxygen consumption and energy expenditure during rest and exercise. Subjects included 60 healthy adults (N = 30 males, N = 30 females) ranging in age from 19 to 65 years (mean +/- SD age, 36.9 +/- 13.4 years) and body mass index (BMI) from 19.2 to 44.8 kg/m2 (27.7 +/- 6.2 kg/m2). Subjects were given two 10 min RMR tests in one test session during which RMR was measured simultaneously with the Douglas bag and FitMate systems. No significant differences were found between Douglas bag and FitMate systems for oxygen consumption (242 +/- 49 and 240 +/- 49 ml/min, respectively, P = 0.066, r = 0.97, mean +/- SD absolute difference 2.83 +/- 11.68 ml/min) or RMR (1,662 +/- 340 and 1,668 +/- 344 kcal/day, P = 0.579, r = 0.97, mean +/- SD absolute difference 5.81 +/- 80.70 kcal/day). These data indicate that the FitMate is a reliable and valid system for measuring oxygen consumption and RMR in adults.