In this randomized controlled trial, we did not find differences in the effect of 2 doses (5mg/kg/day or 15mg/kg/day) of AZM on change in percentage predicted FEV(1), clinical scores, Pseudomonas colonization rates, pulmonary exacerbations and need for antibiotics. There was increase in exacerbations after stopping azithromycin in both the groups. Our results also suggest that the decrease in the incidence of LRTI persists only till 6months after discontinuing azithromycin.