['Cell proliferation, vector titer and accessibility of target cells represent hurdles for efficient gene transfer to lung epithelia in vivo using recombinant murine leukemia (MuLV)-based retroviruses. We tested the hypothesis that the pulmonary epithelium is susceptible to retroviral-mediated gene transfer when stimulated to proliferate by a mitogen, keratinocyte growth factor (KGF).', 'Rats received keratinocyte growth factor (KGF, 2.5 micrograms/g x 4 doses, two consecutive days) intratracheally followed by high titer amphotropic retrovirus expressing beta-galactosidase. Gene transfer was assessed five days later.', 'KGF stimulated transient proliferation in the bronchiolar and alveolar epithelia (30-40% PCNA positive cells at peak) which decreased to background levels seven days after administration. Gene transfer to epithelia (X-Gal positive cells) occurred more frequently in KGF treated rats, but proliferation exceeded the level of gene transfer. X-gal positive cells were noted in the alveolar epithelium and occasionally in the bronchiolar epithelium. In order to understand the discrepancy between the number of proliferating and transduced cells, primary rat tracheal epithelium cultured at the air-liquid interface was infected from either the apical or basolateral side. Gene transfer was achieved only through basolateral application of vector, suggesting that epithelial polarity represents a barrier to MuLV-based lung gene transfer in vivo.'] Does keratinocyte growth factor induced epithelial proliferation facilitate retroviral-mediated gene transfer to distal lung epithelia in vivo?