Xylitol is a 5-carbon sugar that can lower the ASL salt concentration, thus enhancing innate immunity. In this study, the investigators propose to test the safety and tolerability of aerosolized xylitol used daily for 2 weeks in subjects with cystic fibrosis. In a pilot, 2-week study, 60 subjects with cystic fibrosis with an FEV1(Forced expiratory volume in 1 second ) >30% predicted will be randomized to receive aerosolized 7% hypertonic saline (5 ml) or 15% xylitol, (5 ml) twice a day for 14 days. The primary outcomes will be safety as assessed by FEV1 change from baseline, adverse events and respiratory symptom score. Outcomes for trend in efficacy include density of colonization of sputum, time to next exacerbation, sputum cytokines and revised CF quality of life questionnaire.