To evaluate the potential of direct transfer of cystic fibrosis transmembrane conductance regulator (CFTR) cDNA for the treatment of cystic fibrosis (CF), we administered an E1-deficient adenovirus, encoding CFTR, to a defined area of nasal airway epithelium of three individuals with CF. This treatment corrected the Cl⁻ transport defect that is characteristic of CF-affected epithelia. After treatment, there was a decrease in the elevated basal transepithelial voltage, and the normal response to a cAMP agonist was restored. We found no evidence of viral replication or virus-associated adverse effects, even at the highest dose tested (25 MOI). These data represent a small step in achieving long-term improvement of CF lung function by gene therapy.