Increased energy expenditure, poor dietary intake, and fat malabsorption in patients with cystic fibrosis (CF) frequently lead to growth failure and malnutrition, which are associated with pulmonary failure and decreased survival. The study purpose was to understand better the energy expenditure and requirements in the mild pulmonary disease state in children. Resting and total energy expenditure were measured in 6- to 9-yr-old, pancreatic-insufficient children with CF (n = 25) and control children (n = 25) of similar age, gender, and weight. The effect of the most common genotype, homozygous delta F508, on energy expenditure was also investigated. Dietary intake, degree of fat malabsorption, body composition, physical activity, and clinical status were determined. The CF group had a 9% increase in resting energy expenditure, which was not related to genotype or severity of lung disease. Both CF genotype subgroups (delta F508 homozygous and all others) had a similar, modest resting energy expenditure increase. Total energy expenditure was increased by 12% in the entire CF group and by 23% in the delta F508 homozygous CF subgroup compared with controls. The total energy expenditure increase in delta F508 homozygous children may be related to increased voluntary physical activity, reflecting no activity reduction associated with lung disease, or to an unidentified genotype-related mechanism. The clinical implication is that a detailed physical activity assessment should be evaluated along with resting energy expenditure, either measured or estimated by equations, when daily energy needs are being determined for children with CF.