Resting energy expenditure was measured by open-circuit indirect calorimetry in 71 patients, aged 8.9 to 35.5 years, with cystic fibrosis who had no recent history of acute lung infection. Pulmonary function and nutritional status were studied simultaneously. In most patients, resting energy expenditure was above normal (range 95% to 153% of predicted values for age, sex, and weight as derived from the Harris Benedict equations), and was negatively correlated with pulmonary function (P less than 0.01) and nutritional status (P less than 0.01) when expressed as a percentage of body fat. Pulmonary status was positively correlated with nutritional status (P less than 0.01). We conclude that resting energy expenditure in patients with cystic fibrosis exceeds normal values and that the increase correlates with a deterioration in lung function and nutritional status.