Cystic fibrosis (CF) is a common, fatal recessive disease caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene manifested by abnormalities in the regulation of chloride ion (Cl-) secretion across the apical membrane of epithelial cells throughout the body. Adenovirus-mediated delivery of the normal CFTR cDNA and correction of the CF epithelial cell Cl- secretory phenotype suggests the feasibility of gene therapy for CF lung disease. Few studies, however, have focused on the evaluation of the safety of the adenovirus-mediated gene transfer approach. This study presents in vitro data on the efficacy and safety of adenovirus-mediated transfer of the human CFTR cDNA using Av1Cf2. Av1Cf2-mediated transfer of the human CFTR cDNA complemented the abnormal cAMP-regulated Cl- permeability of cells with the CF epithelial phenotype. Av1 vectors did not replicate infectious virus in HeLa cells infected in vitro, although trace vector DNA synthesis was observed at very high multiplicity of infection. Expression of the adenoviral late gene for the hexon capsid protein was observed at trace levels in Av1 vector-infected HeLa cells, but not in freshly isolated human bronchial epithelial cells, consistent with the pattern of DNA synthesis observed in these different target cells. Although, these observations support the efficacy and safety of use of Av1Cf2 for treatment of the fatal pulmonary component of CF.