The long-term survival of lung cancer patients treated with conventional therapies (surgery, radiation therapy and chemotherapy) remains poor and has changed little in decades. The need for novel approaches remains high and gene therapy holds promise in this area. A number of genes have been shown in vitro, in animal studies and most recently, in human clinical trials, to have antitumor actions. However, a number of problems still exist and success in human patients to date has been marginal. Among the numerous considerations are the efficiency of delivery of the gene to the tumor or, if an indirect effect is the aim, possibly nontumor tissues, the efficiency and persistence of expression of the therapeutic gene, the specificity of the gene action against the tumor, potential toxic or pathogenic consequences of either the genes or the delivery vectors used, convenience of the therapy and how likely the therapy will compliment or complicate other conventional anticancer therapies. After the cloning of the cystic fibrosis gene, there was great interest in the noninvasive delivery of genes directly to the pulmonary surfaces by aerosol. Clearly, this approach could have application to some pulmonary cancers as well and most early efforts focused mainly on the use of nonviral vectors, primarily cationic lipids. Unfortunately, nebulization shear forces and inefficient pulmonary uptake and expression of plasmid DNA-cationic lipid formulations have generally resulted in a lack of therapeutic effect, so much of this work has diminished in recent years. Polyethyleneimine (PEI)-based formulations have proven stable during nebulization and result in nearly 100% efficient transfection throughout the airways and lung parenchyma. Therapeutic responses have been obtained in several animal lung tumor models when PEIbased formulations of p53 and other antitumor genes were delivered by aerosol. In addition, this mode of delivery seems to be associated with low toxicity and results in little or none of the immunostimulatory response typically associated with the delivery of bacterially produced plasmid DNA containing unmethylated CpG motifs, which has presented a challenge to repeated gene therapy via other modes of delivery. Other potential applications of PEI aerosol gene delivery include the treatment of asthma, lung alveolitis and fibrosis and a variety of monogeneic diseases such as cystic fibrosis and alpha-1-antitrypsin deficiency. In addition, a wide range of conditions treatable via genetic immunization could benefit from this approach to gene delivery as well.