Cystic fibrosis (CF) lung disease reflects the failure of airways defense against chronic bacterial infection. Studies of CF cultures, transgenic mice, and CF patients suggest that the initiating event in CF airways disease pathogenesis is reduced airway surface liquid (ASL) volume, i.e., dehydration. CF ASL volume regulation depends on a single extracellular signaling system, ATP, which renders CF airways more vulnerable to disease-causing insults (e.g., viruses) than are normal airways, which regulate ASL volume by dual ATP and adenosine signaling pathways. Clinical studies have explored the hypothesis that treating the dehydration of CF airways will be therapeutically beneficial. Inhaled hypertonic saline osmotically draws water onto airway surfaces, improves mucus clearance and pulmonary function, and reduces acute exacerbations in CF patients. Thus, rehydration therapies may slow the progression of CF lung disease in patients with established bacterial infection and may prevent the onset of CF lung disease if initiated early in life.