Excess neutrophils are present in the airways of patients with cystic fibrosis (CF). Myeloperoxidase (MPO) activity of acid extracts of sputum is directly correlated with airflow obstruction in CF patients. We hypothesized that the sputum MPO was derived from the MPO of neutrophils that entered the airways from the circulation. Active MPO without protease activity injures airways. If MPO activity from circulating neutrophils that emigrate into the airways of these patients causes increased airway epithelial permeability and mucus-gland secretion, then (1) those patients with greater MPO activity per circulating neutrophil would be more likely to produce sputum and (2) the MPO activity per circulating neutrophil would positively correlate with airflow obstruction. We determined the MPO activity for both circulating and sputum neutrophils. Spirometry and respiratory cultures were obtained simultaneously with blood and sputum samples. CF patients with more MPO activity within their circulating neutrophils were more likely to produce sputum ( P =.001, chi 2 test), and the MPO activity per circulating neutrophil was positively correlated with airflow obstruction as measured on the basis of the ratio of 1-second forced expiratory volume to forced vital capacity ( P <. 03, Kruskal-Wallace test). These associations were independent of age, sex, the results of respiratory-tract culture, or protease activity in the circulating neutrophils. MPO activity in circulating neutrophils from CF patients homozygotic for the deletion of phenylalanine at position 508 in the CF transmembrane regulator protein is directly related to the severity of these patients' pulmonary disease. Our results are consistent with the hypothesis that circulating neutrophils deliver active MPO to the airway, producing airway injury and airflow obstruction in homozygotic delF508 CF patients.