Recombinant human deoxyribonuclease (rhDNase) has been shown to reduce sputum viscoelasticity and to improve lung function in patients with cystic fibrosis (CF). The aim of this study was to determine whether airway inflammation would decrease after administration of rhDNase. Twenty patients with CF and chronic suppurative lung disease inhaled 2.5 mg of rhDNase daily for 1 month. Before and after the 1-month trial, lung function was measured and sputum was obtained, either after spontaneous expectoration or after sputum induction with hypertonic saline. Sputum total cell and differential counts were measured using techniques previously described. The mean age of the patients was 16.8 years (range, 6.7-27.5). After 1 month of rhDNase, mean FEV1 increased from a baseline of 62.3% predicted to 70.8% (P= 0.02, paired t test); and FVC increased from 74.4% to 83.9% predicted (P=0.007). No significant differences were found in sputum cytology before or after rhDNase (median total cell counts 16.0 x 10(6)/ml vs. 19.3 x 10(6)/ml, P=0.68). Thirteen patients had a 10% or greater increase in FEV1 after rhDNase (responders). Initial lung function was less in responders than in nonresponders (53.5% vs. 78.6%, P=0.007). There was no significant change in total cell count and neutrophil count after rhDNase in either responders or nonresponders. We conclude that airway inflammation, as measured by total cell counts in sputum, was a prominent feature in cystic fibrosis, and neutrophils were the dominant inflammatory cells. Although the administration of rhDNase resulted in significant improvements in FEV1, there was no evidence of accompanying changes in airway inflammation.