Increased energy expenditure associated with active inflammation has been thought to be one cause of weight loss in patients with Crohn's disease. Our aim was to test this hypothesis by determining if resting energy expenditure (REE) measured by indirect calorimetry was greater than the predicted energy expenditure (PEE) calculated from the Harris-Benedict formula (variables--sex, age, height, and weight) in each patient. Fifty-four patients with radiographic evidence of Crohn's disease were studied. There was a highly significant relationship (p less than 0.001) between REE and PEE, which can be expressed as follows: REE = 39.40 + 0.99 (PEE). The mean REE was 1427 +/- 228 kcal/day, whereas the mean PEE was 1404 +/- 197 kcal/day. Patients with the lowest weights when expressed as percentages of ideal body weights had the greatest resting energy expenditure per kilogram of body weight (r = -0.73, p less than 0.001). The mean REE per kilogram per day was 25 +/- 4 kcal, and only 4 of 54 patients (7%) had REE greater than or equal to 30 kcal/kg X day. Thus, REE measured by indirect calorimetry in Crohn's disease patients was not significantly higher than PEE that can be estimated from the Harris-Benedict equation. These findings show that most Crohn's disease patients without fever or sepsis do not have increased REE.