Cystic fibrosis (CF) results from mutations of the CF transmembrane conductance regulator (CFTR) gene and subsequent defective regulation of cAMP-stimulated chloride (Cl-) permeability across the apical membrane of epithelial cells. In vitro transfer of normal CFTR cDNA corrects this defect, and studies in experimental animals have shown successful gene transfer to airway epithelium in vivo using a recombinant adenoviral vector containing the human CFTR cDNA (AdCFTR), supporting the feasibility of in vivo AdCFTR-mediated gene therapy for the respiratory manifestations of CF. One step in applying this therapy to CF patients is to evaluate the safety and efficacy of AdCFTR-mediated gene transfer in the actual target for human gene therapy, human airway epithelium. The present study demonstrates that AdCFTR restores cAMP-stimulated Cl- permeability in human CF bronchial epithelial cells. In addition, the study utilizes freshly isolated human airway epithelial cells from the nose and/or bronchi of normal individuals and/or individuals with CF to demonstrate that after in vitro AdCFTR-mediated gene transfer: (i) AdCFTR DNA does not replicate as a function of dose and time; (ii) CF epithelial cells express AdCFTR-mediated normal human CFTR mRNA; and (iii) CF epithelial cells, including terminally differentiated ciliated cells (the most common airway epithelial cell type), express the normal human CFTR protein. Together, these data support the use of AdCFTR in human gene therapy trials and suggest that biologic efficacy should be achievable in vivo.