Study objective: The 12-week efficacy and safety of aerosolized recombinant human DNase (dornase alfa) were evaluated in previously untreated patients with cystic fibrosis (CF) with advanced lung disease. Design: In this multicenter, double-blind, placebo-controlled study, CF patients with advanced lung disease were randomized to receive either dornase alfa or placebo once a day for 12 weeks. Patients: A total of 320 patients in clinically stable condition with documented CF and an FVC less than 40% of predicted were recruited from 65 CF Foundation care centers in the United States. The dornase alfa and placebo groups were comparable with respect to age (range, 7 to 57 years), height, and weight, Male subjects outnumbered female subjects (55% vs 45%) and few subjects were younger than 17 years of age (15%), The percentages of predicted FEV(1) and FVC were significantly lower in the dornase alfa group at baseline (p less than or equal to 0.05). Interventions: Patients were randomly assigned to receive either 2.5 mg dornase alfa once daily (n=158) or placebo once daily (n=162). All patients continued to receive standard medications and treatments administered for CF. Measurements and results: Dornase alfa improved the mean percent change in FEV(1) from baseline by 9.4% compared with 2.1% for placebo (p<0.001). The actively treated group showed a 12.4% improvement in FVC compared with 7.3% for placebo (p<0.01). There were no differences between the treatment groups in dyspnea score, number of days receiving IV antibiotics, or length of hospital stay; the overall incidence of adverse events was comparable between treatment groups, Fifteen patients died: 9 in the dornase alfa group and 6 in the placebo group; no differentiating clinical characteristics were demonstrated. Conclusions: Pulmonary function as measured by FEV(1) and FVC improved significantly in the dornase alfa-treated patients, Dornase alfa was found to be safe and well tolerated over the 12-week study period.