The aim of the present study was: (1) to develop a new method for total energy expenditure (TEE) assessment, using accelerometry (ACC) and heart rate (HR) telemetry in combination; (2) to validate the new method against the criterion measure (DLW) and to compare with two of the most common methods, FLEX-HR and ACC alone. In the first part of the study VO2, HR and ACC counts were measured in twenty-seven subjects during walking and running on a treadmill. Considering the advantages and disadvantages of the HR and ACC methods an analysis model was developed, using ACC at intensities of low and medium levels and HR at higher intensities. During periods of inactivity, RMR is used. A formula for determining TEE from ACC, HR and RMR was developed: TEE =1·1×(EQHR×TTHR+EQACC1×TTACC1+EQACC2×TTACC2+RMR×TTRMR). In the validation part of the study a sub-sample of eight subjects wore an accelerometer, HR was logged and TEE was measured for 14d with the DLW method. Analysis of the Bland–Altman plots with 95% CI indicates that there are no significant differences in TEE estimated with HR–ACC and ACC alone compared with TEE measured with DLW. It is concluded that the HR–ACC combination as well as ACC alone has potential as a method for assessment of TEE during free-living activities as compared with DLW