Background and Aims: The objective of the study was to compare data obtained from the Cosmed K4 b 2 and the Deltatrac II T M metabolic cart for the purpose of determining the validity of the Cosmed K4 b 2 in measuring resting energy expenditure.Methods: Nine adult subjects (four male, five female) were measured. Resting energy expenditure was measured in consecutive sessions using the Cosmed K4 b 2 , the Deltatrac II T M metabolic cart separately and the Cosmed K4 b 2 and Deltatrac II T M metabolic cart simultaneously, performed in random order. Resting energy expenditure (REE) data from both devices were then compared with values obtained from predictive equations.Results: Bland and Altman analysis revealed a mean bias for the four variables, REE, respiratory quotient (RQ), V CO 2 , V O 2 between data obtained from Cosmed K4 b 2 and Deltatrac II T M metabolic cart of 268+/-702 kcal/day, -0.0+/-0.2, 26.4+/-118.2 and 51.6+/-126.5 ml/min, respectively. Corresponding limits of agreement for the same four variables were all large. Also, Bland and Altman analysis revealed a larger mean bias between predicted REE and measured REE using Cosmed K4 b 2 data (-194+/-603 kcal/day) than using Deltatrac T M metabolic cart data (73+/-197 kcal/day).Conclusions: Variability between the two devices was very high and a degree of measurement error was detected. Data from the Cosmed K4 b 2 provided variable results on comparison with predicted values, thus, would seem an invalid device for measuring adults.