Cystic fibrosis (CF) is characterized by the production of abnormally thick secretions in the airways, chronic bacterial endobronchial infections and a chronic, predominantly neutrophilic inflammatory response. Therefore, myeloperoxidase (MPO) and lactoferrin are frequently used as inflammatory markers. Recently, a new protein in the neutrophil granules, human neutrophil lipocalin (HNL) has been discovered. The aim of the present study was to investigate HNL in sera of patients with CF and its relation to MPO and lactoferrin as well as to acute pulmonary exacerbation. Serum concentrations of HNL, MPO and lactoferrin were determined in 42 patients with CF and in 25 healthy subjects. Patients with CF were divided into groups with and without acute pulmonary exacerbation (APE) and also with and without colonization with Pseudomonas aeruginosa (Pa). Median serum levels of HNL (200.5 microg x L(-1)), MPO (595 microg x L(-1)) and lactoferrin (1,356.5 microg x L(-1)) were significantly increased in patients with CF compared to control subjects (57.7, 178 and 478 microg x L(-1), respectively; p<0.0001). CF patients with APE had significantly increased serum concentrations of HNL (321 versus 97.7 microg x L(-1); p<0.0001), MPO (1,125 versus 300 microg x L(-1); p<0.005) and lactoferrin (4,936 versus 980 microg x L(-1); p<0.001) compared with patients in stable clinical condition. Similarly, patients colonized with Pa had significantly higher concentrations of HNL, MPO and lactoferrin than Pa negative patients. These results indicate that in patients with cystic fibrosis, serum concentrations of human neutrophil lipocalin are markedly increased with a strong relationship to myeloperoxidase and lactoferrin. Thus, determination of serum human neutrophil lipocalin concentrations may be another useful diagnostic tool to monitor neutrophil inflammation in cystic fibrosis. The more marked difference in human neutrophil lipocalin compared with myeloperoxidase concentrations with no overlap between patients with acute pulmonary exacerbation and those in stable condition even suggests that human neutrophil lipocalin may be a more sensitive and specific discriminator.