Cystic Fibrosis (CF) is an inherited lethal genetic disorder resulting in recurrent respiratory infections and progressive lung disease with a current life expectancy of 36 years. This clinical trial will use 'azithromycin' - an antibiotic with potent anti-inflammatory effects - to prevent early lung damage. Following newborn diagnosis, children will receive either azithromycin three times per week or matched placebo with all receiving standard CF care. Success will be measured by the absence of bronchiectasis, a type of lung scarring, on chest CT scans at age 1 and 3 years, with additional measures of infection and inflammation in the lower airways. Chest CT scans and airway lavage are routine care in many CF clinics. By targeting treatment in infancy, we hope to prevent lung damage, improve quality of health and gain years of life in this condition for which, at this time, there is no cure.