OBJECTIVES—Mesenchymal stem cells (MSCs) are one of a number of cell types undergoing extensive investigation for cardiac regeneration therapy. It has not yet been determined whether this cell therapy also substantially contributes to vascular remodeling of diseased vessels. METHODS AND RESULTS—Human MSCs and a variety of progenitor and vascular cells were used for in vitro and in vivo experiments. Wire-induced vascular injury mobilized MSCs into the circulation. Compared with human aortic smooth muscle cells, MSCs exhibited a 2.8-fold increase in the adhesion capacity in vitro (P<0.001) and a 6.3-fold increase in vivo (P<0.001). In all animal models, a significant amount of MSCs contributed to intimal hyperplasia after vascular injury. MSCs were able to differentiate into cells of endothelial or smooth muscle lineage. Coculture experiments demonstrated that late-outgrowth endothelial cells (OECs) guided MSCs to differentiate toward an endothelial lineage through a paracrine effects. In vivo, cell therapy with OECs significantly attenuated the thickness of the neointima contributed by MSCs (intima/media ratio, from 3.2±0.4 to 0.4±0.1, P<0.001). CONCLUSIONS—Tissue regeneration therapy with MSCs or cell populations containing MSCs requires a strategy to attenuate the high potential of MSCs to develop intimal hyperplasia on diseased vessels.