Background: Aortic aneurysms are found in 5-20% of patients with acute aortic dissection (AAD).Coexisting aortic aneurysms might potentially influence the incidence of AAD. The purpose of this study was to elucidate the role of coexistent aortic aneurysms in AAD. Methods and Results: A total of 140 patients with AAD were enrolled in the present study. Clinical characteristics of the patients were evaluated in relation to the locations of aortic segments affected by the dissection as well as of the coexistent aortic aneurysm. Among the 140 study patients, 34 (24%) had true aortic aneurysms. Patients with coexistent aortic aneurysm were significantly older than those without (72±11 years vs 65±14 years, P=0.012) and had higher incidence of thrombosed false lumen (62% vs 38%, P=0.017), and coronary artery disease (26% vs 8%, P=0.006). Twenty-two of these 34 (65%) patients had a thoracic aortic aneurysm (TAA), and this frequency of TAA was much higher than that observed in the general population. Furthermore, among all patients with AAD, 12 patients (9%) might be associated with development of AAD. Conclusions: The current study showed that nearly one-quarter of AAD patients had coexisting true aortic aneurysms, and suggests that TAA are likely to be associated with development of AAD. (Circ J 2009; 73: 822 -825)