To evaluate the clinical characteristics and outcomes of patients with painless acute aortic dissection (AAD). For this study conducted from 1997 to 2001, we searched the International Registry of Acute Aortic Dissection to identify patients with painless AAD (group 1). Their clinical features and in-hospital events were compared with patients who had painful AAD (group 2). Of the 977 patients in the database, 63 (6.4%) had painless AAD, and 914 (93.6%) had painful AAD. Patients in group 1 were older than those in group 2 (mean +/- SD age, 66.6 +/- 13.3 vs 61.9 +/- 14.1 years; P = .01). Type A dissection (involving the ascendIng aorta or the arch) was more frequent in group 1 (74.6% vs 60.9%; P = .03). Syncope (33.9% vs 11.7%; P < .001), congestive heart failure (19.7% vs 3.9%; P < .001), and stroke (11.3% vs 4.7%; P = .03) were more frequent presenting signs in group 1. Diabetes (10.2% vs 4.0%; P = .04), aortic aneurysm (29.5% vs 13.1%; P < .001), and prior cardiovascular surgery (48.1% vs 19.7%; P < .001) were also more common in group 1. In-hospital mortality was higher in group 1 (33.3% vs 23.2%; P = .05), especially due to type B dissection (limited to the descending aorta) (43.8% vs 10.4%; P < .001), and the prevalence of aortic rupture was higher among patients with type B dissection in group 1 (18.8% vs 5.9%; P = .04). Patients with painless AAD had syncope, congestive heart failure, or stroke. Compared with patients who have painful AAD, patients who have painless AAD have higher mortality, especially when AAD is type B.