 - This is a single center, randomized, double-blinded, placebo-controlled 24 week trial of azithromycin in adults with CF chronically infected with BCC. The investigational therapy will be added to usual therapy in patients who are clinically stable at the time of enrollment. After 168 days, the study drug will be discontinued and participants will be evaluated at 196 days off of study drug for 28 days. At that visit, participants will be invited to continue in an open label observational study of azithromycin for 168 additional days. Thus, the duration of the study will be 52 weeks 364 days. 