Cystic fibrosis (CF) lung disease reflects persistent bacterial infection of airway lumens. Several hypotheses have been advanced to link mutations in the CFTR gene to the failure of the CF lung to defend itself against bacterial infection. Amongst the most productive hypotheses at present is the ''low airway surface liquid (ASL) volume'' or ''dehydration'' hypothesis. This hypothesis predicts that airway surface dehydration produces the mucus adhesion, inflammation, and bacterial biofilm formation characteristic of CF. Clinical trials of inhaled hypertonic saline have demonstrated therapeutic benefit of manoeuvres designed to rehydrate CF airway surfaces.