Xylitol, a five-carbon sugar with low transepithelial permeability, which is poorly metabolized by bacteria can lower the salt concentration of both cystic fibrosis (CF) and non-CF epithelia in vitro. Xylitol is an artificial sweetener that has been successfully used in chewing gums to prevent dental caries; it has been used as an oral sugar substitute without significant adverse effects. It has also been shown to decrease the incidence of acute otitis media by 20-40%; nasal application to normal human subjects was found to decrease colonization with coagulase negative staphylococcus. We found that aerosolized iso-osmolar xylitol was safe in mice, healthy volunteers and stable subjects with CF when administered over a single day. In a recent study, we observed that single doses of 10% followed by 15% xylitol was well tolerated by subjects with cystic fibrosis who were stable. In this pilot study we propose to test the hypothesis that aerosolized hypertonic xylitol given daily for 2 weeks, will be safe and well tolerated and potentially lower the density of colonization in subjects with CF compared to hypertonic saline. We chose hypertonic concentration of xylitol to be comparable in part to hypertonic saline which is being offered as a routine treatment in hospitalized patients with CF exacerbation.