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Various pulmonary disorders, including cystic fibrosis, are potentially amenable to a treatment modality in which a therapeutic gene is directly delivered to the lung. Current gene delivery systems, either viral or nonviral, need further improvement in terms of efficiency and safety. We reported that nonionic amphiphilic block copolymers hold promise as nonviral gene delivery systems for transfection of muscular tissues. To evaluate the efficiency of these vectors in the lung, intratracheal instillation or aerosolization of reporter genes complexed with Lutrol or PE6400 was performed. Lutrol–DNA and, to a lesser extent, PE6400–DNA complexes promoted efficient gene transfection into mouse airways in a dose-dependent manner. This improvement over naked DNA was observed irrespective of the reporter gene. Lutrol enabled us to deliver significantly higher DNA amounts than current nonviral vectors, with even greater increases in gene expression and without the formation of colloidally unstable complexes. Time course studies showed that Lutrol–DNA complexes permitted prolonged gene expression for up to 5 days whereas with poly(ethylenimine) (PEI)–DNA polyplexes, expression peaked on days 1–2 postinstillation, was strongly reduced by day 5, and reached background levels on day 7. Aerosolized delivery of Lutrol–DNA complexes, a less invasive approach to deliver genes to the lung, gave 5- to 15-fold higher reporter gene expression compared with PEI–DNA polyplexes administered via the same delivery route. After intratracheal instillation of Lutrol–DNA complexes, histochemical staining for β-galactosidase expression showed the presence of large blue areas. Histopathological analysis showed that Lutrol alone did not elicit inflammation, and that the inflammatory response after intratracheal instillation of Lutrol–DNA complexes was reversible and was observed only with the highest amounts of DNA. We also found that Lutrol can efficiently deliver genes to the airways of cystic fibrosis mice. Thus, we conclude that Lutrol is a highly promising vector for gene delivery to the lung.