Airway surface liquid (ASL) is an integral part of lung defense mechanisms. Ion transport by airway epithelia regulates the volume and composition of this fluid. A better understanding of the mechanisms of ion transport will enable the development of new therapies for airway diseases associated with defects in these mechanisms. A useful model of a disease with abnormal airway epithelial ion transport is cystic fibrosis (CF), a distinct genetic syndrome of altered lung defense mechanisms characterized by chronic bacterial infection and a steady decline in lung function. Traditional therapies for CF include antibacterial drugs and augmentation of clearance of secretions, but investigators are now studying pharmacological approaches to target the more basic defect of the disease, i.e. abnormal sodium and chloride ion transport. Early treatment in childhood, prior to lung damage, might prevent or at least retard the decline in pulmonary function that remains the hallmark of CF. Ion transport dysfunction may also contribute to other airway diseases such as asthma and chronic bronchitis. Pharmacological intervention at this level may prove beneficial in these common lung diseases as well.