Inclusion Criteria
Documented diagnosis of cystic fibrosis
At least two respiratory cultures growing P. aeruginosa within the last 12 months
Use of at least two cycles of inhaled tobramycin within the last 24 weeks
Off TISP and other inhaled anti-pseudomonal antibiotics for at least 2 weeks at Visit 1 and remain off of any inhaled antibiotics for an additional 2 weeks before starting inhaled tobramycin
Prior or current use of azithromycin for at least four consecutive weeks
Exclusion Criteria
Positive pregnancy test, lactating, or unwillingness to practice a pre-defined form of contraception, which includes abstinence
Inability or unwillingness to cycle off of inhaled tobramycin for one 4-week period and without Use of any additional inhaled antibiotics
Use of intravenous or oral anti-pseudomonal antibiotics within 4 weeks of screening
Use of investigational therapy within 4 weeks of screening
Use of systemic corticosteroids equivalent to a daily dose more than 10mg of prednisone
Use of nelfinavir, warfarin, haloperidol, or methadone (concern of drug interaction with azithromycin) modulator therapy within 30 days
Any other condition that, in the opinion of the site investigator, would compromise the safety of the subject or quality of the data