Due to their quiescent nature and spatial complexity, many target tissues for gene therapy will require novel strategies. An alternative to ex vivo gene transfer, providing many technical advantages and possibly allowing sufficient transfer of the therapeutic gene, is direct in vivo delivery of the vehicle. For a favorable outcome, this procedure is dependent on a high-titer vector, fully competent before post-mitotic cells. In view of the restrictions inherent in the use of retroviruses, we have investigated the potentials of adenovirus. Adenoviruses have as primary targets of infection the differentiated epithelial cell. The large DNA genome of the virus hints to a large cloning capacity. Furthermore, wildtype adenovirus and the precedent of administration to humans are suggestive of adenovirus-based gene therapy for diseases involving a variety of quiescent tissues. The use of a replication-defective adenovirus carrying a gene encoding a nuclearly-targeted beta-galactosidase demonstrated that replication-incompetent adenoviruses offer an efficient means to transfer a gene for extended periods of time to the liver, muscle, lung, and brain. Because of adenovirus' natural tropism for the lung epithelium, we have proposed that this virus be used as a CFTR gene delivery vector for the treatment of cystic fibrosis and lung cancer.