To evaluate the clinical characteristics, risk factors, and outcomes of hypotension in unselected patients who had acute aortic dissection (AAD), we studied 1,073 such patients who were enrolled in the International Registry of Acute Aortic Dissection between 1996 and 2001. Hypotension was noted in 313 patients (29.2%) who had AAD (46.0% on admission). Multivariate logistic regression identified age >or=70 years (odds ratio [OR] 2.0, 95% confidence interval [CI] 1.4 to 2.9), type A dissection (referent type B AAD; OR 2.1, 95% CI 1.4 to 3.2), neurologic deficit (OR 3.8, 95% CI 2.2 to 6.6), syncope (OR 2.9, 95% CI 1.8 to 4.7), aortic regurgitation requiring valve surgery (OR 1.9, 95% CI 1.1 to 3.3), cardiac tamponade (OR 5.1, 95% CI 3.0 to 8.8), and new Q-wave or ST-segment deviation on an electrocardiogram (OR 1.6, 95% CI 1.1 to 2.4) as independent associations of hypotension (c statistic 0.78). Hospital complications (neurologic deficits 22.7% vs 12.0%, altered mental status 26.1% vs 4.4%, myocardial ischemia 14.6% vs 6.9%, mesenteric ischemia 6.9% vs 2.6%, or limb ischemia 14.6% vs 6.9%, and death 55.0% vs 10.3%) occurred more frequently in patients who had hypotension than in those who did not (p <0.001 for all comparisons). We concluded that hypotension that occurred in >25% of patients who had AAD was associated with a much higher rate of in-hospital adverse events. Our study also identified factors associated with hypotension in patients who had AAD.