Rationale : Loss of fibulin-4 during embryogenesis results in perinatal lethality because of aneurysm rupture, and defective elastic fiber assembly has been proposed as an underlying cause for the aneurysm phenotype. However, aneurysms are never seen in mice deficient for elastin, or for fibulin-5, which absence also leads to compromised elastic fibers. Objective : We sought to determine the mechanism of aneurysm development in the absence of fibulin-4 and establish the role of fibulin-4 in aortic development. Methods and Results : We generated germline and smooth muscle cell (SMC)-specific deletion of the fibulin-4 gene in mice ( Fbln4 GKO and Fbln4 SMKO , respectively). Fbln4 GKO and Fbln4 SMKO aortic walls fail to fully differentiate, exhibiting reduced expression of SM-specific contractile genes and focal proliferation of SMCs accompanied by degenerative changes of the medial wall. Marked upregulation of extracellular signal-regulated kinase 1/2 signaling pathway was observed in the aneurysmal wall of Fbln4 GKO and Fbln4 SMKO mice and both mutants developed aneurysm predominantly in the ascending thoracic aorta. In vitro, Fbln4 GKO SMCs exhibit an immature SMC phenotype with a marked reduction of SM-myosin heavy chain and increased proliferative capacity. Conclusions : The vascular phenotype in Fbln4 mutant mice is remarkably similar to a subset of human thoracic aortic aneurysms caused by mutations in SMC contractile genes. Our study provides a potential link between the intrinsic properties of SMCs and aneurysm progression in vivo and supports the dual role of fibulin-4 in the formation of elastic fibers as well as terminal differentiation and maturation of SMCs in the aortic wall.