The ActiReg (PreMed AS, Oslo, Norway) system is unique in using combined recordings of body position and motion alone or combined with heart rate (HR) to calculate energy expenditure (EE) and express physical activity (PA). The ActiReg has two pairs of position and motion sensors connected by cables to a battery-operated storage unit fixed to a waist belt. Each pair of sensors was attached by medical tape to the chest and to the front of the right thigh respectively. The collected data were transferred to a personal computer and processed by a dedicated program ActiCalc. Calculation models for EE with and without HR are presented. The models were based on literature values for the energy costs of different activities and therefore require no calibration experiments. The ActiReg system was validated against doubly labelled water (DLW) and indirect calorimetry. The DLW validation demonstrated that neither EE calculated from ActiReg data alone (EEAR) nor from combined ActiReg and HR data (EEAR-HR) were statistically different from DLW results. The EEAR procedure causes some underestimation of EE >11 MJ corresponding to a PA level >2.0. This underestimation is reduced by the EEAR-HR procedure. The objective recording of the time spent in different body positions and at different levels of PA may be useful in studies of PA in different groups and in studies of whether recommendations for PA are being met. The comparative ease of data collection and calculation should make ActiReg a useful instrument to measure habitual PA level and EE.