Accumulation of viscous sputum enhances lung damage in patients with cystic fibrosis. High contents of DNA, released by leucocytes, are the major cause of the high viscosity of this sputum. Recombinant human DNase I (rhDNase) decreases viscoelastic properties of sputum in cystic fibrosis and improves lung function. We have investigated the effect of rhDNase over a period of 6 weeks, 2.5 mg once daily, on lung function and quality of life in 12 adult patients with cystic fibrosis. Significant improvements in FEV1, IVC and daily peak flow values were measured. Airway resistance and FVC did not improve significantly. The cause of the improvements found is probably due to recruitment of previously, due to mucus plugging, unventilated lung areas. In addition, the awareness of symptoms related to sputum retention improved, although subjective parameters on general well-being and physical condition remained unchanged.