Complex interactions between nutrition, skeletal and respiratory muscle function and energy expenditure in cystic fibrosis patients exist. Malnutrition significantly contributes to muscle weakness in patients with chronic obstructive pulmonary disease of the adult or in cystic fibrosis in childhood. Together with a measurable increase in resting energy expenditure the malnutrition, as a consequence of pancreatic insufficiency, leads to pulmonary deterioration. Whether pulmonary disease, pancreatic insufficiency, increased energy expenditure or insufficient intake of nutrition are the starters for the destructive circle or whether the basic defect is responsible for some of the components interacting with each other remains to be determined. Copyright 2000 Harcourt Publishers Ltd.