Recombinant human DNase I, or dornase alfa, is the first new therapy developed specifically for cystic fibrosis in almost 30 years. It selectively digests extracellular DNA and reduces the viscosity of purulent sputum. In clinical trials dornase alfa modestly improved pulmonary function, slightly decreasing the number of respiratory exacerbations requiring parenteral antibiotics compared with placebo. Phase III studies suggest that patients receiving dornase alfa also spend slightly fewer days in the hospital than those treated with placebo. The aerosolized preparation is safe and generally well tolerated. Voice alteration and sore throat are the most commonly reported adverse effects. Further research is necessary to determine the optimum time to initiate therapy and to evaluate the agent's pharmacoeconomic impact on the treatment of cystic fibrosis. Aerosolized dornase alfa should always be given in conjunction with standard cystic fibrosis therapies including antibiotics, chest physiotherapy, and pancreatic enzyme supplementation.