A 50-year-old man was admitted to our institution for upper abdominal pain. The diagnosis was chronic pancreatitis. A pseudocyst 3鈥塩m in diameter had been detected in the head of the pancreas by ultrasonography a year and a half earlier. At the current hospitalization, ultrasound examination showed a mass 5鈥塩m in diameter where the pseudocyst had been seen. This mass comprised an external hypoechoic lesion, a middle movable hyperechoic lesion, and an internal cystic lesion showing an arterial signal on Doppler ultrasound examination. Computed tomography showed a high-density area thought to be a hematoma in the head of the pancreas, and extravasation in the mass was demonstrated by rapid injection of a contrast medium. Our diagnosis was a bleeding pseudocyst. The patient suddenly vomited fresh blood and went into hypovolemic shock on the 11th hospital day. Emergency angiography and transcatheter embolization with steel coils was accomplished under a diagnosis of rupture of the bleeding pseudocyst into the duodenum. Ultrasonographic examination showed that the mass had decreased in size and the cystic lesion, showing an arterial signal, had disappeared after embolization. Although bleeding pseudocysts occur infrequently, they are a life-threatening complication of chronic pancreatitis. Early diagnosis and treatment are thus essential. Ultrasound examination with Doppler ultrasonography is strongly indicated when diagnosing bleeding pseudocysts and examining patients with chronic pancreatitis accompanied by abdominal pain, gastrointestinal bleeding, or both. Early treatment by transcatheter embolization should be used to avoid this potentially lethal complication.