 Despite the inflammatory nature of lung disease in CF, nitric oxide NO formation as well as the expression of NOS2 has been found to be decreased in CF airways. While the reasons for impaired airway NO formation remain incompletely understood, there is evidence that low NO formation contributes to lung pathophysiology in CF. Constitutive endogenous formation of Nitric oxide NO in airways is thought to play a role in neurotransmission, smooth muscle relaxation and bronchodilation. Previous animal experiments have shown that the addition of L-arginine, the precursor of enzymatic NO formation, resulted in a significantly greater relaxation of tracheas. There is also evidence that a single dose of inhaled L-arginine improves pulmonary function in CF. In this study we will assess the effect of L-arginine inhalation on lung function, nitric oxide formation, airway inflammation and bacterial infection in CF patients. 