Free-living energy expenditure was estimated by doubly-labelled water (DLW) and continuous heart-rate (HR) monitoring over nine consecutive days in nine healthy men with sedentary occupations but different levels of leisure-time physical activity. Individual calibrations of the HR-energy expenditure (EE) relationship were obtained for each subject using 30 min average values of HR and EE obtained during 24 h whole-body calorimetry with a defined exercise protocol, and additional data points for individual leisure activities measured with an Oxylog portable O2 consumption meter. The HR data were processed to remove spurious values and insert missing data before the calculation of EE from second-order polynomial equations relating EE to HR. After data processing, the HR-derived EE for this group of subjects was on average 0.8 (SEM 0.6) MJ/d, or 6.0 (SEM 4.2) % higher than that estimated by DLW. The diary-respirometer method, used over the same 9 d, gave values which were 1.9 (SEM 0.7) MJ/d, or -12.1 (SEM 4.0) % lower than the DLW method. The results suggest that HR monitoring can provide a better estimate of 24 h EE of groups than the diary-respirometer method, but show that both methods can introduce errors of 20% or more in individuals.