Crohn's disease is an inflammatory process commonly characterized by phases of flare-up. Weight loss and malnutrition are prominent features in the course of the disease, especially during acute episodes. It is therefore important to define energy needs. Curiously, resting energy expenditure (REE) has rarely been studied in Crohn's disease, and never in relation with the activity of the disease. We therefore determined REE together with body composition (fat free mass, FFM), Crohn's disease activity index (CDAI) and plasma acute phase proteins in 70 patients: during flare-up in 41 and during clinical remission (CDAI < 150) in 29. We found an increase in REE in patients with active disease (CDAI > 150), as compared with patients in remission, when REE was expressed as a function of FFM: 31.7 +/- 2.7 versus 29.4 +/- 3.3 kcal/kg FFM/day (P < 0.01). The mean REE/FFM was 8% higher during flare-up than during remission, and was correlated to both clinical (CDAI; P = 0.011) and biological inflammatory activity indices (C reactive protein, P = 0.018; orosomucoid, P = 0.024). In some patients, the REE was in the normal range, despite an increase in REE/FFM, because of a decrease in FFM due to hypermetabolism. In 8 patients treated successfully by total parenteral nutrition for a massive flare-up, REE/FFM was increased before TPN (36.6 +/- 3.0 kcal/kg/day), and decreased after 4 weeks of TPN (31.4 +/- 1.8 kcal/kg/day; P < 0.001), returning within normal values in 7 patients.