O2-uptake was measured continuously in 18 polytraumatized and 21 long-term ventilated patients. All patients were on assisted ventilation with sufficient sedation and analgesia. O2-uptake was measured every minute, the values were recorded over 24 h and the mean O2-uptake/ min was calculated. At the same time, the O2-uptake of the last 15 min of every hour was measured separately (e.g. 7.45–8.00, 8.45–9.00, etc.). The mean O2-uptake/min measured continuously was compared and correlated with the mean O2-uptake per day, calculated on the basis of each of the short-term measurements. There were only slight differences between the mean values of the two procedures: In the group of long-term ventilated patients the mean O2-uptake was found to be 364 ml/min in continuous measurements and 363 ml/min in short-term measurements. This O2-uptake corresponded to an energy expenditure of 2360 kcal/day. The mean correlation coefficient was 0.89 (range: 0.79–0.96). If O2-uptake was measured over 2 periods of 15 min each, e.g. in the late morning and in the afternoon, the mean correlation coefficient improved to 0.94 and the standard deviation was reduced. Comparable results were obtained in the group of polytraumatized patients. This study shows that under certain preconditions short-term measurements of O2-uptake of 2 × 15 min allow sufficiently reliable predictions of the daily O2-uptake and energy expenditure in severely traumatized or critically ill pa