To determine whether recombinant human growth hormone (rhGH), glutamine (GLN) or a combination of both agents can enhance protein synthesis in cystic fibrosis (CF) patients, six 9.6 ± 0.5-year-old prepubertal children (4 M, 2 F) with CF and stable lung disease with undernutrition (weight/height 13C-leucine were used to assess leucine appearance rate (Ra, an index of protein breakdown), oxidation (Ox), and non-oxidative leucine disposal (NOLD, an index of protein synthesis). Results are expressed as changes (%) from baseline:We conclude that in children with CF: (1) due to high inter-subject variability, oral glutamine does not significantly enhance protein gain; (2) rhGH has significant anabolic effects which are mediated by stimulation of protein synthesis, and (3) glutamine does not enhance the effect of rhGH.