Energy expenditure during various activities of daily living in normally nourished female adolescents with cystic fibrosis was compared with that in matched healthy control subjects. Energy expenditure at rest, during sitting and standing, and during two levels of exercise was increased significantly in patients with cystic fibrosis (122% +/- 14%) compared with control subjects (104% +/- 10%) (p < 0.05), but incremental increases from one level of activity to another did not differ. We conclude that the various activities of daily living are not responsible for increased energy needs in female adolescents with cystic fibrosis.