The effect of refeeding on resting energy expenditure (REE) and substrate utilization was studied in 18 hospitalized adolescent girls (aged 12.9-19.1 years) suffering from anorexia nervosa. Changes in body composition were monitored weekly and included weight, fat body mass (FBM), lean body mass (LBM) and total body potassium (TBK). REE was studied weekly by open-circuit calorimetry. Weight gain was noted in all patients (38.2 +/- 5.6 to 44.5 +/- 5.3 kg), involving increased FBM and LBM. REE increased per kg of weight (91.6 +/- 15.1 to 101.7 +/- 18.0 kJ kg-1 d-1) and LBM over the first weeks of refeeding (P less than 0.025) and then stabilized. Substrate utilization showed an increase in carbohydrate and protein utilization (P less than 0.001) during the first few weeks of refeeding. We also studied the thermic effect of food (TEF) in 14 of the 18 subjects. Upon admission the subjects had a reduced TEF (36.4 +/- 24.3 kJ 2 h-1) (P less than 0.001). With refeeding TEF rose to a peak or plateau, then decreased to normal levels (61.9 +/- 36.0 kJ 2 h-1) before discharge from hospital. We conclude that the energy metabolism of adolescent girls adapts to semi-starvation by a reduction in both REE and TEF; with refeeding there is reversal of this adaptive function.