The authors encountered a case of portal-systemic venous shunt newly diagnosed after initiation of hemodialysis. A 68-year-old Japanese woman began hemodialysis because of symptoms of uremia including loss of appetite and pulmonary edema. Loss of consciousness occurred suddenly after her ninth session of hemodialysis. No hepatic functional abnormality was found other than hyperammonemia (314 microg/dL [184 micromol/L]). Loss of consciousness subsequently occurred often after hemodialysis. Color Doppler ultrasonography and magnetic resonance angiography depicted a large shunt between the left gastric vein and left renal vein resulting in portal flow entering the systemic circulation via the renal vein. Because the shunt was large, ligation of it was performed surgically. Results of histologic examination of a liver biopsy specimen obtained intraoperatively were normal. The patient became well postoperatively. This patient's encephalopathy appeared to be caused by the flow of ammonia-rich portal venous blood into the systemic circulation via the large shunt owing to a decrease in intravenous pressure after rapid hemodialysis. Portal-systemic shunt encephalopathy should be recognized as a "new" neuropsychiatric disorder characteristic of patients undergoing hemodialysis.