The purpose of this study is to evaluate whether Biaxin (clarithromycin) improves sputum abnormalities, lung function, and overall feeling of well-being in people with cystic fibrosis (CF). Biaxin is an antibiotic commonly used for the treatment of respiratory infections in people who do not have CF, and is sometimes used in CF patients as well. Studies done in a disease called diffuse panbronchiolitis (which is similar to CF) and some preliminary studies that have been done in CF patients suggest that Biaxin might have a beneficial effect on CF sputum in ways unrelated to its antibiotic activity.

Inclusion Criteria:

• diagnosis of CF
• ability to reliably perform spirometry
• FEV1 of 30% predicted at the time of screening

Exclusion Criteria:

• Mycobacterium in a sputum culture ever recorded
• a respiratory exacerbation requiring IV antibiotics in the 60 days prior
• used a investigational drug or device in the 60 days prior
• significant (>30ml) of hemoptysis in the past year
• require oxygen or have significant liver or renal disease