Previous studies have indicated that medical therapy provides excellent outcomes for patients with uncomplicated Stanford type B acute aortic dissection. However, affected aortas are often compromised by aneurysmal dilatation and rupture, resulting in poor outcomes. The purpose of this study was to determine predictors of aortic events in patients with Stanford type B acute aortic dissection receiving conservative medical therapy. The study group consisted of 78 consecutive patients with Stanford type B acute aortic dissection who were admitted to the hospital within 48 hours of onset. These patients were treated medically and followed up for 1 year. Aortic events were defined as rupture, recurrent dissection, aortic expansion with diameter>or=60 mm, rapid aortic expansion at a rate of >or=10 mm/yr, and the development of visceral or limb ischemia. Predictors of these events were determined using multivariate analyses. During 1-year follow-up, aortic events were observed in 13 (17%) patients, including aortic rupture in 3 (4%), aortic diameter>or=60 mm in 4 (5%), rapid expansion of the aorta in 3 (4%), and the development of visceral or limb ischemia in 3 (4%). On multivariate analysis, fibrinogen-fibrin degradation product level>or=20 microg/ml (odds ratio 7.802, 95% confidence interval 1.405 to 43.335) on admission was the only independent predictor of aortic events at 1 year. In conclusion, careful monitoring is required for patients with medically treated Stanford type B acute aortic dissection associated with fibrinogen-fibrin degradation product level>or=20 microg/ml on admission.