Resting energy expenditure (REE) was measured by indirect calorimetry and body composition was assessed by both direct (bioimpedance) and indirect (anthropometry) methods in 20 hospitalized patients with biopsy-proven ileal Crohn's disease and in a group of 16 healthy volunteers matched for sex, age, and height with the patient group. The Crohn's disease activity index was below 120 in all patients studied, who were treated with a low dose of corticosteroids (0.2-0.3 mg/kg body wt of prednisone). The average weight of Crohn's patients was significantly lower than that of controls (55.70 vs 70.50 kg, P < 0.001) due to both lower fat mass (9.97 vs 18.30 kg, P < 0.001) and lower lean body mass (45.72 vs 52.20 kg, P < 0.02). The average REE was significantly higher in the control group (1785.42 +/- 7.503 vs 1559.1 +/- 48.39 kcal/day, P < 0.001). However, these differences disappeared when REE was normalized by lean body mass (LBM) (34.49 +/- 2.56 vs 34.704 +/- 3.75 kcal/kg LBM P = NS). The nonprotein respiratory quotient was significantly lower in the patient group (0.823 +/- 0.031 vs 0.882 +/- 0.012, P < 0.025), indicating an increased lipid oxidation. This increased lipid oxidation might explain the reduced fat stores found in the group of Crohn's patients, suggesting also that a sufficiently lipid-rich diet could be useful in their nutritional management.