We hypothesize that inhaled mannitol 400 mg b.i.d. will increase the mean change from baseline FEV1 (mL) compared to control over the 26-week treatment period in adult subjects with cystic fibrosis. Any improvement in FEV1 is considered clinically meaningful, however, this trial has set a threshold of 80 mL for the purposes of determining an appropriate sample size for statistical power while retaining trial feasibility in an orphan disease population