Vascular smooth muscle cell (VSMC) proliferation and migration contribute significantly to atherosclerosis, postangioplasty restenosis, and transplant vasculopathy. Forkhead transcription factors belonging to the FoxO subfamily have been shown to inhibit growth and cell cycle progression in a variety of cell types. We hypothesized that forkhead proteins may play a role in VSMC biology. Under in vitro conditions, platelet-derived growth factor (PDGF)-BB, tumor necrosis factor-alpha, and insulin-like growth factor 1 stimulated phosphorylation of FoxO in human coronary artery smooth muscle cells via MEK1/2 and/or phosphatidylinositol 3-kinase-dependent signaling pathways. PDGF-BB, tumor necrosis factor-alpha, and insulin-like growth factor 1 treatment resulted in the nuclear exclusion of FoxO, whereas PDGF-BB alone down-regulated the FoxO target gene, p27(kip1), and enhanced cell survival and progression through the cell cycle. These effects were abrogated by overexpression of a constitutively active, phosphorylation-resistant mutant of the FoxO family member, TM-FKHRL1. The anti-proliferative effect of TM-FKHRL1 was partially reversed by small interfering RNA against p27(kip1). In a rat balloon carotid arterial injury model, adenovirus-mediated gene transfer of FKHRL1 caused an increase in the expression of p27(kip1) in the VSMC and inhibition of neointimal hyperplasia. These data suggest that FoxO activity inhibits VSMC proliferation and activation and that this signaling axis may represent a therapeutic target in vasculopathic disease states.