Background: Combining accelerometry (ACC) with heart rate (HR) monitoring is thought to improve activity energy expenditure (AEE) estimations compared to ACC alone. Objective: To evaluate the validity of ACC and HR used alone or combined to estimate AEE in free living conditions, compared to doubly-labeled water (DLW). Methods: Ten-day free-living AEE was measured by a DLW protocol in thirty-five 18-55 y men (11 lean active, 12 lean sedentary, 12 overweight sedentary) wearing an Actiheart (combining ACC and HR) and a RT3 accelerometer. AEE was estimated using group- or individual-calibration of the HR/AEE relationship, based on an exercise tolerance test. In a subset (n=21), AEE changes were measured after one month of detraining (active subjects) or an 8-week training (sedentary subjects). Results: Actiheart combined ACC/HR estimates were more accurate than estimates from HR or ACC alone. Accuracy of the Actiheart group-calibrated ACC/HR estimates was modest (intra-class correlation coefficient [ICC]=0.62), with no bias but high root mean square error (RMSE) and limits of agreement (LOA). The mean bias of the estimates was reduced by one third, like RMSE and LOA, by individual calibration (ICC=0.80). Contrasting with group-calibrated estimates, the Actiheart individual-calibrated ACC/HR estimates explained 38% of the variance of the DLW-AEE change (ICC=0.63). Conclusion: This study supports a good level of agreement between the Actiheart ACC/HR estimates and DLW-measured AEE in lean and overweight men with varying fitness levels. Individual calibration of the HR/AEE relationship is necessary for AEE-estimations at individual level rather than at group scale and for AEE change evaluation.