Abstract—Matrix remodeling plays an important role in the physiological and pathological remodeling of blood vessels. We specifically investigated the role of matrix metalloproteinase (MMP)-9, an MMP induced during arterial remodeling, by assessing the effects of genetic MMP-9 deficiency on major parameters of arterial remodeling using the mouse carotid artery flow cessation model. Compared with remodeling of matched wild-type (WT) arteries, MMP-9 deficiency decreased intimal hyperplasia, reduced the late lumen loss, eliminated the correlation between intimal hyperplasia and geometric remodeling, and led to significant accumulation of interstitial collagen. Biochemical analysis of MMP-9 knockout (KO) arterial tissue and isolated smooth muscle cells (SMCs) confirmed the lack of MMP-9 expression or compensation by other gelatinases. To investigate potential mechanisms for the in vivo observations, we analyzed in vitro effects of MMP-9 deficiency on the migration, proliferation, and collagen gel contracting capacity of aortic SMCs isolated from MMP-9 KO and WT mice. Although proliferation was comparable, we found that MMP-9-deficient cells had not only decreased migratory activity, but they also had decreased capacity to contract collagen compared with WT cells. Thus, MMP-9 appears to be involved not only in degradation, but also in reorganization of a collagenous matrix, both facets being essential for the outcome of arterial remodeling. Our results also establish MMP-9 as an attractive therapeutic target for limiting the effects of pathological arterial remodeling in restenosis and atherosclerosis.