Background. Thoracic aortic aneurysmal diseases are characterized by degeneration of elastin within the aortic wall. Although proteinases, such as matrix metalloproteinase, appear to contribute to elastin degradation, little is known about the role of elastic fiber assembly in such diseases. Fibulin-5 is an extracellular protein that is expressed in the vascular basement membrane and regulates elastic fiber assembly by microfibril machinery. In this study, we examined whether thoracic aortic dissection (TAD) is associated with abnormal fibulin-5 expression. Methods. Intraoperative aortic samples were obtained from 21 patients with proximal aortic dissection. Control aortic tissue was obtained from 11 organ donors, heart transplant recipients, and patients undergoing coronary artery bypass. An in vitro culture of vascular smooth muscle cells was obtained from 2 TAD patients and 1 control subject. To evaluate elastin expression, we stained tissue sections with Verhoeff-Van Gieson stain. Fibulin-5 messenger RNA (mRNA) expression was determined by quantitative real-time reverse-transcriptase-polymerase chain reaction. Results. Aortic fibulin-5 mRNA and elastin content were decreased in TAD patients, compared with controls (P = .001 and P = .02, respectively). Decreased fibulin-5 expression strongly correlated with decreased amounts and fragmentation of elastin in aortic samples from patients with TAD (r = 0.83, P < .0001 and F = 20.7, P < .0001 respectively). The fibulin-5 mRNA in aortic vascular smooth muscle cells collected from TAD demonstrated a 38% decrease in expression, compared with the control. Conclusions. Patients with proximal aortic dissection exhibited significantly decreased expression of aortic fibulin-5. Decreased fibulin-5 may contribute to the pathogenesis of aortic dissection by impairing elastic fiber assembly.