Objective estimates of activity patterns and energy expenditure (EE) are important for the measurement of energy balance. The Intelligent Device for Energy Expenditure and Activity (IDEEA) can estimate EE from the thirty-five postures and activities it can identify and record. The present study evaluated the IDEEA system's estimation of EE using whole-body indirect calorimetry over 24 h, and in free-living subjects using doubly-labelled water (DLW) over 14 d. EE was calculated from the IDEEA data using calibration values for RMR and EE while sitting and standing, both as estimated by the IDEEA system (IDEEA(est)) and measured by indirect calorimetry (IDEEA(meas)). Subjects were seven females and seven males, mean age 38.1 and 39.7 years, mean BMI 25.2 and 26.2 kg/m(2), respectively. The IDEEA(est) method produced a similar estimate of EE to the calorimeter (10.8 and 10.8 MJ, NS), while the IDEEA(meas) method underestimated EE (9.9 MJ, P<0.001). After removing data from static cycling, which the IDEEA was unable to identify as an activity, both the IDEEA(est) and IDEEA(meas) methods overestimated EE compared to the calorimeter (9.9 MJ, P<0.001; 9.1 MJ, P<0.05 and 8.6 MJ, respectively). Similarly, the IDEEA system overestimated EE compared to DLW over 14 d; 12.7 MJ/d (P<0.01), 11.5 MJ/d (P<0.01) and 9.5 MJ/d for the IDEEA(est), IDEEA(meas) and DLW, respectively. The IDEEA system overestimated EE both in the controlled laboratory and free-living environments. Using measured EE values for RMR, sitting and standing reduced, but did not eliminate, the error in estimated EE.