The association between abnormal chloride transport, resulting from mutations in the cystic fibrosis transmembrane regulator (CFTR) gene, and the immunologic processes involved in the development of CF lung disease is poorly understood. However, neutrophil-dominated inflammation on the respiratory epithelial surface is a common finding in CF patients and suggests a mechanism for the immunologic abnormalities described in CF. Of particular importance for the pathophysiology of CF are proteases such as neutrophil elastase (NE) which are released from neutrophils in CF airways and cause direct structural damage to respiratory tissue. In healthy individuals, the deleterious effects of excess protease activity in the respiratory system are inhibited by antiproteases such as alpha 1-antitrypsin (alpha 1AT) and secretory leukoprotease inhibitor (SLPI). However, in CF, antiproteases are outnumbered by proteases and this protective mechanism is rendered ineffective. Restoration of this protease/antiprotease balance through antiprotease replacement therapy is currently under clinical investigation and preliminary results are promising.