The present invention is directed to a method for treating cystic fibrosis. The method comprises the steps of: identifying a patient suffering from cystic fibrosis, and administering to the patient an effective amount of denufosol or a pharmaceutically acceptable salt thereof and an effective amount of a macrolide. In one method, denufosol and the macrolide are administered by inhalation, preferably in a single formulation. In another method, denufosol is administered by inhalation and the macrolide is administered orally. The present invention is also directed to a pharmaceutical formulation comprising denufosol or a pharmaceutically acceptable salt thereof, a macrolide, and a pharmaceutically acceptable carrier. Preferred denufosol is denufosol tetrasodium and preferred macrolide is azithromycin. The pharmaceutical formulation preferably is in a form of an inhalable dry powder or in a liquid form.