Progressive pulmonary infection is the dominant clinical feature of cystic fibrosis (CF), but the molecular basis for this susceptibility remains incompletely understood. To study this problem, we developed a model of chronic pneumonia by repeated instillation of a clinical isolate of Burkholderia cepacia (genomovar III, ET12 strain), an opportunistic gram-negative bacterium, from a case of CF into the lungs of Cftr (m1unc-/-) (Cftr(-/-)) and congenic Cftr(+/+) controls. Nine days after the last instillation, the CF transmembrane regulator knockout mice showed persistence of viable bacteria with chronic severe bronchopneumonia while wild-type mice remained healthy. The histopathological changes in the lungs of the susceptible Cftr(-/-) mice were characterized by infiltration of a mixed inflammatory-cell population into the peribronchiolar and perivascular spaces, Clara cell hyperplasia, mucus hypersecretion in airways, and exudation into alveolar airspaces by a mixed population of macrophages and neutrophils. An increased proportion of neutrophils was observed in bronchoalveolar lavage fluid from the Cftr(-/-) mice, which, despite an increased bacterial load, demonstrated minimal evidence of activation. Alveolar macrophages from Cftr(-/-) mice also demonstrated suboptimal activation. These observations suggest that the pulmonary host defenses are compromised in lungs from animals with CF, as manifested by increased susceptibility to bacterial infection and lung injury. This murine model of chronic pneumonia thus reflects, in part, the situation in human patients and may help elucidate the mechanisms leading to defective host defense in CF.