The potential of DNase I to increase cystic fibrosis sputum elastase activity and lung damage was evaluated. Sputum from CF patients induced little lung hemorrhage when instilled intranasally in C57BL/6 mice. However, sputum treated in vitro by the addition of 1 mg/ml bovine DNase I showed increased neutrophil elastase activity (7.97 ± 1.56 versus 3.91 ± 0.62 μ M, p < 0.01) and induced marked lung hemorrhage in mice (bronchoalveolar lavage fluid hemoglobin = 192.8 ± 40.7 versus 44.5 ± 12.0 μ g/ml, p < 0.01). These effects were not observed with DNase I alone in phosphate buffer and were suppressed by the human neutrophil elastase inhibitor methoxysuccinyl-alanyl-alanyl-prolyl-valine-chloromethylketone (MeOSAAPV-CMK). In vivo administration of 2.5 mg aerosolized recombinant human DNase I to patients with CF resulted in a 2.2-fold increase of sputum elastase activity within 1 h of treatment. Elastase levels returned to pre-rhDNase therapy levels 24 h after aerosol treatment. Sputum collected 1 h after rhDNase on 4 separate days from two of six patients in which elastase levels were highest, induced lung hemorrhage when instilled intranasally in mice. We conclude that DNase I therapy of patients with cystic fibrosis can acutely increase the elastase activity of sputum and also its potential to induce hemorrhage in the murine lung.