 Previous work demonstrated that inhaled hypertonic saline HS reduces exacerbation frequency and improves lung function in adults with cystic fibrosis CF. It is unclear, however, whether HS will benefit young patients suffering from CF. The investigators propose to further support the concept that HS can benefit children with mild CF lung disease by performing a relatively short, placebo controlled study of HS in 5-12 year olds, using lung function and mucociliary clearance as key outcome measures. 