a. Less than 2 acute exacerbations in the preceding 12 months and none in the preceding 4 weeks, with acute exacerbation defined as persistent (>24 hour) worsening of >3 respiratory symptoms associated with bacterial infection including: i. Cough ii. Dyspnea iii. Hemoptysis iv. Increased sputum purulence or volume v. Chest pain b. No change in antimicrobial regimen for at least 3 months before enrollment c. FEV1 80% predicted d. Use of short-acting beta-agonist 2 days/week for at least the past 3 months e. No baseline requirement for oxygen supplementation f. Ability to maintain oxyhemoglobin saturation of 90% during and after 6-minute walk test g. Not currently taking oral corticosteroids