The resting energy expenditure (REE) and substrate oxidation rates in 16 patients with cystic fibrosis who had mild chest disease and 11 healthy controls were measured using indirect calorimetry. The mean REE (% predicted) in the patients with cystic fibrosis was 11% greater than in the controls. Five patients with cystic fibrosis were hypermetabolic but only one of these had a clinically significant reduction of respiratory function. A greater proportion of REE was derived from carbohydrate oxidation in the cystic fibrosis patients (43.5% v 29.9%). However, the 24 hour dietary intake of carbohydrate was greater in the cystic fibrosis group (49.6 v 45.8% of energy intake). These data suggest that a high dietary intake of carbohydrate may contribute to the increased oxidation of carbohydrate in these cystic fibrosis patients. All patients with cystic fibrosis, including those with apparently mild lung disease, should continue to receive a high energy diet.