We are conducting this research study to examine the safety, effect on lung function and frequency of symptoms relating to cystic fibrosis during 24 weeks of treatment with the antibiotic azithromycin. This study is designed to determine if patients with cystic fibrosis whose lungs are not infected with the bacteria Pseudomonas aeruginosa will benefit from 24 weeks of treatment with the antibiotic azithromycin. Benefit will be determined as having better pulmonary function tests and getting sick less often compared to a placebo (sugar pill). This study is also designed to determine if azithromycin is safe when administered for 24 weeks to cystic fibrosis patients not infected with Pseudomonas aeruginosa. By doing this study, we hope to learn more about CF and improve the way in which we treat it.