A Study to Determine the Variability of a 6-Minute Walk Test in Cystic Fibrosis Subjects With Advanced Lung Disease

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