Fourteen patients aged 4.9 to 21.5 years with cystic fibrosis and moderate to severe lung disease, malnutrition, or growth failure were given nocturnal supplemental feeding by gastrostomy tube. Mean follow-up was for 1.1 years (range 0.8 to 2.78 years). Patients were studied to observe the effect of nutritional support on body composition, growth, pulmonary function, and quality of life. A contemporary group of patients with CF was retrospectively pair matched to the study group. The supplemental feeding resulted in positive changes in body composition and in growth velocity. Weight, as a percentage of standard in the control group, declined by 3% over 1 year, whereas it increased by 2% in the treatment group (P less than 0.05). Pulmonary function, assessed as a percent of predicted FVC and FEV1, did not change significantly in the treatment group over 1.1 years, whereas FVC declined by 12% (P less than 0.01) and FEV1 declined by 13% (P less than 0.01) in the control group. There was a marked increase in patient ability to participate in activities of daily living, even in those patients in whom pulmonary function deteriorated during the study.