The energy needs, nutritional status and body composition of 6 undernourished adolescents and young adults with cystic fibrosis (3 male and 3 female) were studied prior to and following a 12 day period of refeeding. Nutritional rehabilitation was attempted with continuous naso-gastric feeding of an elemental diet. The energy needs of the patients were shown to be 25-80% higher than in healthy individuals of the same age, sex and size. Lean body mass as judged by measurement of total body potassium and nitrogen was relatively well preserved, but there was significant wasting of adipose tissue. During the short period of refeeding, body weight, fat and potassium all increased significantly, while fat free body mass and total body nitrogen did not change. A model is proposed for the aetiology of undernutrition in the adolescent and young adult cystic with deteriorating lung function. Based on this model and the finding of the short-term N/G refeeding program described in this paper, we have embarked on a long-term supplementation study in malnourished CF patients who do not respond to dietary counselling and oral supplementation.