Background: Acute aortic dissection (AAD) is a life-threatening condition, and its prompt diagnosis remains essential for successful management. Although sudden and severe pain in the chest, back or abdomen is the most common presenting symptom, some patients with AAD do not present initially with pain. Moreover, there is no data which is associated with painless AAD in Korea. Methods: We analyzed retrospectively 412 patients with AAD from January 1995 to October 2006 in one center. They were divided into patients with painless AAD (group 1) and painful AAD (group 2) to evaluate the clinical characteristics and outcomes of patients with painless AAD. AAD was defined as patient���s presentation within 14 days of onset of symptoms. Results: Among the patients with AAD, 18 (4.4%) had painless AAD, and 394 (95.6%) had painful AAD. Syncope (38.9% vs 7.4%; P<.001) was more frequent presenting sign in group 1. Aortic aneurysm (33.3% vs 15.2%; P=.051) in the past medical history and altered conciousness (22.2% vs 8.6%; P=.073) tended to be more common in group 1. In addition to in-hospital mortality, there were no statistically significant differences between the two groups according to age, prior cardiovascular surgery, past medical history of DM, and dissection type (Stanford type). Of all patients with AAD in the database, because 358 (86.9%) were referred from other hospitals, we could not find out the duration from symptom onset to confirmative diagnosis. Conclusion: This study suggested that painless AAD initially presented with syncope or altered consciousness in the emergency setting and eatablishing diagnosis was difficult especially in the absence of pain. However, actually, most of patients with AAD were referred from other hospitals and then there was no diagnostic delay which made patients with painless AAD more fatal.