Gene therapy studies of cystic fibrosis (CF) have shown that AAV-based vector was efficient in transferring but not in expressing the CFTR cDNA in the target cells. The levels of CFTR gene expression were limited by the small packaging capacity of AAV because it had been difficult to package the CFTR cDNA with an efficient promoter. In the present study we have developed a new generation of AAV/CFTR vectors which contain efficient short promoters to express the CFTR gene in target cells. To do so, we reduced the size of the CFTR cDNA by determining the minimal untranslated regions required for expression of CFTR cDNA. We also identified short and efficient promoters that could be packaged with the down-sized CFTR cDNA into a novel AAV vector that had a maximal packaging capacity. Functional analyses showed that the new vectors were packaged efficiently and expressed higher levels of CFTR than a vector in which the CFTR gene was driven by the ITR sequence of AAV. Transduction of airway epithelial cells containing [symbol: see text] 508 mutation with the new vectors demonstrated efficient expression of the wild-type CFTR and correction of the CF phenotype. In contrast, no significant CFTR expression was detected in cells infected with the vector that express the CFTR gene from the ITR. These findings support the notion that the AAV can be developed into an efficient vector to transduce the CFTR gene and vectors expressing higher levels of CFTR from an efficient promoter should provide better efficacy for gene therapy of cystic fibrosis.