Introduction: Little is known about the optimal timing of rhDNase nebulization in relation to airway clearance therapy (ACT). Objective: To compare the effects of rhDNase before ACT versus rhDNase after ACT in children with CF Methods: Design: randomized, double blind, double dummy, cross over study Inclusion criteria: CF stable clinical condition, rhDNase maintenance therapy Children in Group I inhaled rhDNase 30 minutes before ACT, and placebo directly after ACT in week 1-3. The protocol was reversed during week 4-6. Group II performed the reversed sequence. Patients continued their daily routine ACT Primary endpoint: MEF25 %pred. Pulmonary functions tests were performed on days 0, 14, 21, 35 and 42. In weeks 3 and 6 children scored cough and sputum production on daily diary cards. Results: 24 patients completed the study Mean age = 12 years (range 7-19). Mean MEF25 %pred was 5.8% higher after 3 weeks of rhDNase before ACT, compared to rhDNase after ACT (58.3% vs 52.5%, p = 0.01). There were no significant differences for any of the other variables. Conclusion: Inhalation of rhDNase before ACT improves peripheral airway patency in children with cystic fibrosis. Since all children were already on maintenance rhDNase therapy before the study, this effect is additional to any existing effect of regular rhDNase.