Induced sputum has been used to study airway inflammation. We sought to determine whether markers of infection and inflammation in induced sputum were a useful and safe outcome measure in cystic fibrosis. We hypothesized that bacterial density and inflammatory content of induced sputum would decrease after antibiotic therapy. Induced sputum was assayed for bacterial density, cell count, and differential and inflammatory markers before and after treatment with intravenous antibiotics. Fifty-five of the 72 subjects enrolled (mean age +/- SD 18.2 +/- 7.9 years) completed the study. FEV1 increased by an average 0.3 +/- 0.3 L (10.4 +/- 8.7% predicted FEV1), p<0.0001; density of Pseudomonas aeruginosa and Staphylococcus aureus decreased by 2.4 +/- 3.1 log10 cfu/g (p<0.0005) and 4.0 +/- 2.3 log10 cfu/ml (p<0.0001), respectively; neutrophil count decreased by 0.4 +/- 0.6 log10 cells/ml (p<0.0001), interleukin-8 concentration by 0.5 +/- 1.3 log10 pg/ml (p<0.05), and neutrophil elastase by 0.4 +/- 0.7 log10 microg/ml (p<0.005). Seven of 127 (6%) sputum induction procedures showed a decrease in FEV1 of 20% or more. We conclude that markers in induced sputum may be useful, noninvasive outcome measures to assess response to therapies in cystic fibrosis studies.