Azithromycin is an antibiotic that has been shown to improve lung function in patients with cystic fibrosis (CF) whose lungs are infected with a bacterium called Pseudomonas aeruginosa. Scientists are not sure how azithromycin works in cystic fibrosis. It does not appear to work by killing the bacteria Pseudomonas aeruginosa, but it may make these bacteria and other bacteria less damaging to the lungs by reducing their ability to attach to the lining of the lung, or by reducing the bacteria's ability to make substances that damage the lungs of patients with cystic fibrosis. Azithromycin may also work directly on the cells in the lungs to improve lung function. This could occur by reducing inflammation (swelling) in the lungs, and/or making the mucus less sticky, or by affecting the salt channel that doesn't function correctly in CF. If azithromycin works in one or more of these ways; it may also be effective in improving lung function in cystic fibrosis patients who are not infected with Pseudomonas aeruginosa.