Because interstitial lung disease increases the work of breathing, the aim of this study was to determine if this condition is associated with increased energy requirements. A group of 12 clinically stable patients with interstitial lung disease was studied. Patients with a history of weight loss had significantly more severe lung volume restriction. Regression analysis showed that 42% of body weight variation was explained by vital capacity (p less than 0.025). Resting energy expenditure was measured by standard methods of indirect calorimetry. The measurements were performed with a ventilated hood during prolonged steady-state periods after an overnight fast. We found that resting energy expenditure was increased to 117.3 and 118.7% of the predicted basal metabolic rate, according to Fleisch and to Harris and Benedict reference values, respectively (p less than 0.001). Furthermore, resting energy expenditure was increased to 120.8% of the predicted value according to body fat-free mass (p less than 0.001). This extra energy expenditure in patients with interstitial lung disease is similar to that recently reported in patients with chronic obstructive pulmonary disease.