To determine whether long-term enteral feedings can improve nutritional status and lung function parameters in patients with cystic fibrosis (CF), 11 patients (8 female, 3 male, age 7 to 23 years) received a percutaneous endoscopic gastrostomy (PEG) since February 1988. All patients were moderately to severely affected and extremely malnourished with a relative underweight of 15% or more. After 9 months of supplemental nocturnal feeding, mean body weight improved from 25.8 +/- 4.7 kg (mean, standard deviation) to 31.6 +/- 7.4 kg (p = 0.003); mean relative underweight decreased from 20.7 +/- 3.4% to 10.4 +/- 8.6% (p = 0.02). Lung function, which was measured either prior to or without any intravenous antibiotics, improved considerably: mean vital capacity increased from 39.5% of the predicted value to 46.8% after 9 months (p = 0.01), the mean peak flow rate improved from 39.5% to 50.8% (p = 0.01). This improvement in lung function is probably secondary to the increased muscle mass of the patients. Long-term nocturnal feedings via a percutaneous gastrostomy tube improve nutritional status and lung function severely in malnourished patients with CF.