This is an observational study to be conducted at approximately 25 sites in the United States. Approximately 25 subjects with severe obstructive CF lung disease (FVC <40% predicted) will be enrolled.

Inclusion Criteria:

• Able to provide written informed consent
• Males or females who will be >=14 years old at Visit 3
• A proven diagnosis of CF as evidenced by a positive sweat test (sweat sodium or chloride >60 mEq/L by quantitative pilocarpine iontophoresis), or genotype and clinical symptoms consistent with the diagnosis of CF
• Able to perform reproducible spirometry maneuvers at Visit 1 in accordance with the American Thoracic Society (coefficient of variation for three FEV1 maneuvers <=0.2)
• FVC <40% predicted for height, age and sex at Visit 1
• On a stable regimen of chest physiotherapy (CPT) begun at least 7 days prior to Visit 1
• Able to complete the 6-minute walk test at Visit 1

Exclusion Criteria:

• Use of an investigational drug or device within 28 days prior to Visit 1
• Experiencing an episode of severe acute respiratory failure, as determined by the investigator