G ene transfer using replication-defective adenoviruses (RDAd) holds promise for the treatment of vascular proliferative disorders, but is potentially limited by the capacity of these viruses to infect multiple cell lineages. We have generated an RDAd vector, designated AdSM22-lacZ, which encodes the bacterial lacZ reporter gene under the transcriptional control of the smooth muscle cell (SMC)-specific SM22alpha promoter. Here, we show that in vitro AdSM22-lacZ programs expression of the lacZ reporter gene in primary rat aortic SMCs and immortalized A7r5 SMCs, but not in primary human umbilical vein endothelial cells (HUVECs) or NIH 3T3 cells. Consistent with these results, after intraarterial administration of AdSM22-lacZ to control and balloon-injured rat carotid arteries, beta-galactosidase activity was detected within SMCs of the tunica media and neointima, but not within endothelial or adventitial cells. Moreover, intravenous administration of AdSM22-lacZ did not result in lacZ gene expression in the liver or lungs. Finally, we have shown that direct injection of AdSM22-lacZ into SMC-containing tissues such as the ureter and bladder results in high-level transgene expression in visceral SMCs. Taken together, these results demonstrate that transgene expression after infection with an RDAd vector can be regulated in an SMC lineage-restricted fashion by using a transcriptional cassette containing the SMC-specific SM22alpha promoter. The demonstration of an efficient gene delivery system targeted specifically to SMCs provides a novel means to restrict expression of recombinant gene products to vascular or visceral SMCs in vivo.