BACKGROUND & AIMS: For adequate nutritional support of critically ill children, knowledge of the patient's energy expenditure is required. Steady state measurement by a metabolic monitor are defined as resting energy expenditure and may underestimate total energy expenditure in clinical practise. The aim of this study was to investigate total energy expenditure, resting energy expenditure and the relation with physical activity during critical illness and initial recovery. METHODS: We enrolled 20 patients (0-16yr) with sepsis or following surgery. During the first week following admission, total energy expenditure was measured with doubly labelled water, and compared with daily resting energy expenditure measurements (metabolic monitor). Activity levels were independently determined by tri-axial accelerometry. RESULTS: Resting energy expenditure was not different from Schofield's predicted basal metabolic rate, but was 20% lower than total energy expenditure (P=0.006). Overall physical activity level (=total energy expenditure divided by resting energy expenditure) was 1.22 (95%CI: 1.08-1.36) and activity related energy expenditure (=total energy expenditure minus resting energy expenditure) was associated with accelerometry recordings (R(2)=0.72, P=0.02). CONCLUSIONS: During the week following pediatric intensive care admission, in the individual critically ill patient, activity related energy expenditure should be taken into account to prevent a negative energy balance.