A new, open-circuit, flow-through technique for frequently repeated measurements of energy expenditure is described. It is designed mainly for subjects lying in bed and can be applied to severely injured patients. The measurement will not cause any discomfort to the subject, who does not have to co-operate. The energy expenditure was assessed at +/- 2.6% when the pulse technique was used with ambient air as the calibration gas. No systematic error (95% tolerance limits) was found compared with the conventional Douglas-bag method. The technique described is flexible, because all calculations are performed in a computer connected to the system.