In Crohn's disease, weight loss and malnutrition are very frequently occurring problems. One explanation of these findings could be an increased energy expenditure. Eleven patients were studied, of which six were women and five were men, during a hospital stay for an active episode of Crohn's disease. In three patients the small intestine was affected, in five the colon, and in the last three both the ileum and the colon were affected. Van Hees' activity index on admittance was 196 +/- 52 (132-265). The energy expenditure at rest (ECR) on admittance, was 11% higher than that found in the normal population (p: n.s.). The energy expenditure decreased weekly, in a statistically significant way, during hospitalization. No significant relationship was found between the Van Hees activity index and any of the parameters used to evaluate the energy needs. Those patients whose body weight was less than 90% of the ideal body weight, presented pathologically elevated energy needs when the energy expenditure was expressed in kcal/kg (p = 0.003). Fever was the only analyzed parameter which showed a significant correlation with the changes in energy expenditure: measured by the oxygen consumption index (p = 0.003) and by the percentage of REE (p = 0.006).