Elastin, the main component of the extracellular matrix of arteries, was thought to have a purely structural role. Disruption of elastin was believed to lead to dissection of arteries,, but we showed that mutations in one allele encoding elastin cause a human disease in which arteries are blocked, namely, supravalvular aortic stenosis,. Here we define the role of elastin in arterial development and disease by generating mice that lack elastin. These mice die of an obstructive arterial disease, which results from subendothelial cell proliferation and reorganization of smooth muscle. These cellular changes are similar to those seen in atherosclerosis. However, lack of elastin is not associated with endothelial damage, thrombosis or inflammation, which occur in models of atherosclerosis. Haemodynamic stress is not associated with arterial obstruction in these mice either, as the disease still occurred in arteries that were isolated in organ culture and therefore not subject to haemodynamic stress. Disruption of elastin is enough to induce subendothelial proliferation of smooth muscle and may contribute to obstructive arterial disease. Thus, elastin has an unanticipated regulatory function during arterial development, controlling proliferation of smooth muscle and stabilizing arterial structure.