The ability to mediate gene transfer to the lumen of the lung offers opportunities to treat diseases that affect the airways. Currently, several different gene-transfer vectors are being evaluated for delivery of a variety of therapeutic genes to the airways. These include recombinant viral (adenovirus, adeno-associated virus, retrovirus) and synthetic, self-assembling (cationic lipids, polymers) vectors. Although much of the early focus of these vectors has been directed toward a therapy for cystic fibrosis (1 -5 ), other lung diseases, including asthma, chronic obstructive pulmonary disease, and cancer are being explored as viable disease targets.