This phase IV, multicenter, randomized, open-label, two-period crossover study will evaluate the comparable efficacy and safety of Pulmozyme (dornase alfa) delivered by the eRapid nebulizer system in patients with cystic fibrosis. Patients who have been receiving Pulmozyme once daily chronically for at least 6 months will continue to receive Pulmozyme once daily for a run-in period of 2 weeks using the Pari LC Plus nebulizer. Patients will then be randomized to receive in a crossover design Pulmozyme once daily for two treatment periods of 2 weeks each using either the Pari LC Plus or the eRapid nebulizer. Anticipated time on study treatment is 6 weeks.