Airway nitric oxide production is decreased in cystic fibrosis. As growth hormone therapy has been shown to increase nitric oxide production in growth hormone-deficient patients, it may also affect nitric oxide production in patients with cystic fibrosis. The objective of the present study was to investigate the effect of growth hormone therapy on systemic and airway nitric oxide formation in patients with cystic fibrosis. Nitric oxide metabolites in serum and urine, amino acid concentrations in serum and sputum, as well as exhaled nitric oxide, were measured in children with cystic fibrosis before, during and after 1 yr of treatment with human growth hormone. Nitric oxide metabolite concentrations increased significantly in serum and urine during the treatment period. Serum amino acid concentrations (including l-arginine, the substrate for nitric oxide synthases) also increased during treatment. The systemic bioavailability of l-arginine for nitric oxide synthases, expressed as ratio of l-arginine/l-ornithine+lysine, remained unchanged. In contrast, l-arginine concentrations in sputum decreased significantly during growth hormone treatment, as did exhaled nitric oxide levels. Treatment with growth hormone in children with cystic fibrosis decreases exhaled nitric oxide by reducing the concentration of l-arginine in the airways.