Venous complications in patients with acute pancreatitis typically occur as a form of splenic,portal,or superior mesenteric vein thrombosis and have been detected more frequently in recent reports.Although a well-organized protocol for the treatment of venous thrombosis has not been established,anticoagulation therapy is commonly recommended.A 73-year-old man was diagnosed with acute progressive portal vein thrombosis associated with acute pancreatitis.After one month of anticoagulation therapy,the patient developed severe hematemesis.With endoscopy and an abdominal computed tomography scan,hemorrhages in the pancreatic pseudocyst,which was ruptured into the duodenal bulb,were confirmed.After conservative treatment,the patient was stabilized.While the rupture of a pseudocyst into the surrounding viscera is a well-known phenomenon,spontaneous rupture into the duodenum is rare.Moreover,no reports of upper gastrointestinal bleeding caused by pseudocyst rupture in patients under anticoagulation therapy for venous thrombosis associated with acute pancreatitis have been published.Herein,we report a unique case of massive upper gastrointestinal bleeding due to pancreatic pseudocyst rupture into the duodenum,which developed during anticoagulation therapy for portal vein thrombosis associated with acute pancreatitis. Venous complications in patients with acute pancreatitis typically occur as a form of splenic, portal, or superior mesenteric vein thrombosis and have been detected more frequently in recent reports. Although a well-organized protocol for the treatment of venous thrombosis has not been established, anticoagulation therapy is frequently recommended. A 73-year-old man was diagnosed with acute progressive portal vein thrombosis associated with acute pancreatitis. After one month of anticoagulation therapy, the patient developed severe hematemesis. With endoscopy and an abdominal computed tomography scan, hemorrhages in the pancreatic pseudocyst, which was ruptured into the duodenal bulb, were confirmed. After conservative treatment, the patient was stabilized. Whilst the rupture of a pseudocyst into the surrounding viscera is a well-known phenomenon, spontaneous rupture into the duodenum is rare. Moreover, no reports of upper gastrointestinal bleeding caused by pseudocyst rupture in patients under anticoagul ation therapy for venous thrombosis associated with acute pancreatitis have been published. Herein, we report a unique case of massive upper gastrointestinal bleeding due to pancreatic pseudocyst rupture into the duodenum, which developed during anticoagulation therapy for portal vein thrombosis associated with acute pancreatitis.