Indices of respiratory muscle strength, pulmonary function, and pulmonary diffusing capacity were measured in 11 malnourished children (age 10 to 17 years) with cystic fibrosis, before and after improvement of nutritional status with supplemental parenteral nutrients for 1 month. During this time, the children received 120% of estimated energy requirements (either 3.75% or 22.5% as lipid) and amino acids 2.5 gm/120 kcal by central venous catheter, plus as much of their usual diet as desired. With nutritional supplementation, body weight, triceps skinfold thickness, and mid-arm muscle circumference increased (mean 15%, 62%, and 95%, respectively). Maximum inspiratory airway pressure also increased (mean 29%; P less than 0.01), suggesting improvement in respiratory muscle strength. However, none of the indices of pulmonary function improved. Pulmonary diffusing capacity did not change during parenteral nutrition regardless of the amount of parenteral energy intake supplied by lipid, but arterial oxygen saturation decreased (mean of 93.5% to 91.5%; P less than 0.005). During the month following parenteral nutrition, weight, skinfold thickness, and mid-arm muscle circumference, but not MIP, decreased and arterial oxygen saturation returned to the initial value (P less than 0.01).