rhDNase (Pulmozyme) is a new agent in the therapeutic strategy for patients with cystic fibrosis. It is one of the first specific treatments aimed at the respiratory tract. It affects the extracellular DNA which is present in abundant quantities in the bronchial secretions of these patients. rhDNase significantly reduces the incidence of infections and improves respiratory function. It should be used as a major treatment in combination with all other treatments in patients over 5 years of age with a vital capacity of at least 40% the theoretical value. It is important to schedule the respiratory exercises as a function of rhDNase intake. The long-term therapeutic benefit remains to be evaluated.