A 63-year-old male with atrial fibrillation and mild mitral valve regurgitation was referred to hospital because of a descending aortic aneurysm. During the evaluation, he developed an encephalopathy because of hyperammoniaemia. Further examination revealed a portal systemic shunt, perhaps caused by the noncirrhotic portal hypertension. The patient underwent successful replacement of the aneurysm after controlling the blood ammonia level by eliminating protein from the diet and removal of nitrogen from the gastrointestinal tract. Cardiovascular surgery in a patient with noncirrhotic portal hypertension and a portal systemic shunt has not been previously reported. Meticulous management of the perioperative blood ammonia concentration is essential.