Background. Aortic arch surgery for patients with acute aortic dissection is frequently complicated by excessive bleeding and transfusion of allogeneic blood products. However, the physiopathology of acute aortic dissection and surgery-induced coagulopathy has never been precisely studied. The aim of the present study is to describe the changes of the perioperative hemostatic system in patients with acute aortic dissection undergoing aortic arch surgery. Methods. Sixty-two patients undergoing emergent aortic arch surgery for Stanford type A acute aortic dissection were enrolled in this study from January 2013 to September 2014. The hemostatic system was evaluated using standard laboratory tests, plasma fibrinogen levels, and thromboelastogragh at 5 time points: anesthesia induction (T0), lowest nasopharyngeal temperature (T1), protamine reversal (T2), 4 h after surgery (T3), and on the first postoperative day (T4). Results. The study results revealed that clotting factors had a tendency to be consumed in the preoperative period. Surgery and hypothermia resulted in a progressive reduction in clotting factors, platelet counts, and function, as well as fibrinogen concentration and function. After hemostatic therapy, although platelet counts were constantly low, clotting factors and platelet function returned to nearly preoperative levels. In contrast, fibrinogen concentration and function were still significantly lower than preoperative levels. Conclusions. The results of this prospective analysis showed that acute aortic dissection itself activated the hemostatic system even before surgery. After hemostatic therapy, fibrin formation was more impaired than platelet function. In this setting, we proposed that hemostatic therapy should focus on rapid and sufficient supplementation of fibrinogen. Thus, we recommend further increases in fibrinogen concentration to improve coagulopathy in patients with acute aortic dissection. (C) 2016 by The Society of Thoracic Surgeons