The liver X receptors alpha and beta (LXRalpha and LXRbeta) are important regulators of cholesterol homeostasis in liver and macrophages. Synthetic LXR ligands prevent the development of atherosclerosis in murine models; however, the potential functional relevance of LXRs in vascular smooth muscle cells (VSMCs) has not been investigated. In the present study, we demonstrate that LXRs are expressed and functional in primary human coronary artery VSMCs (CASMCs). LXR ligands inhibited mitogen-induced VSMC proliferation and G1-->S phase progression of the cell cycle. Inhibition of G1 exit by LXR ligands was accompanied by a dose-dependent inhibition of retinoblastoma protein (Rb) phosphorylation, which functions as the key switch for G1-->S cell cycle progression. LXR ligands suppressed mitogen-induced degradation of the cyclin-dependent kinase inhibitor p27Kip1, attenuated cyclin D1 and cyclin A expression, and inhibited the expression of S phase-regulatory minichromosome maintenance protein 6. Stabilization of p27kip1 by LXR ligands was mediated by supressing the transcriptional activation of the S phase kinase-associated protein 2 (Skp2), an F-box protein that targets p27Kip1 for degradation. Inhibition of Rb phosphorylation and G1-->S cell cycle progression by LXR ligands was reversed in VSMCs overexpressing Skp2, indicating that Skp2 as an upstream regulator of p27Kip1 degradation plays a central role in LXR ligand-mediated inhibition of VSMC proliferation. Furthermore, adenovirus-mediated overexpression of the S phase transcription factor E2F, which is released after Rb phosphorylation, reversed the inhibitory effect of LXR ligands on VSMC proliferation and S phase gene expression, suggesting that the primary mechanisms by which LXR ligands inhibit VSMC proliferation occur upstream of Rb phosphorylation. Finally, neointima formation in a model of rat carotid artery balloon injury was significantly attenuated after treatment with the LXR ligand T1317 compared with vehicle-treated animals. These data demonstrate that LXR ligands inhibit VSMC proliferation and neointima formation after balloon injury and suggest that LXR ligands may constitute a novel therapy for proliferative vascular diseases. The full text of this article is available online at http://circres.ahajournals.org.