We present a case of portal-systemic encephalopathy due to a congenital splenorenal shunt. A 69 year old woman was admitted to hospital because of recurrent episodes of disturbed consciousness. The present episode had begun 3 months prior to admission. Although the patient demonstrated mildly slurred speech, the remainder of her neurological examination was unremarkable. She showed no clinical signs of portal hypertension and her liver function, except for a serum hepaplastin test of 58% and an ICG retention rate of 28% at 15 min, was normal. Her serum ammonium level was 210 micrograms/dL. The venous phase of a superior mesenteric arteriogram revealed a splenorenal shunt and narrowing of the portal vein, which was 4 mm in diameter. The histological findings, demonstrated by a needle liver biopsy specimen, were consistent with mild fibrosis and lymphocytic infiltration. Following the diagnosis of a splenorenal shunt in the absence of liver cirrhosis, ligature of the shunt was performed with a splenectomy. The portal vein pressure after ligature of the shunt rose from 12.5 to 18.8 mmHg. This shunt was thought to be of congenital origin. The high preoperative serum ammonia concentration decreased to the normal range postoperatively and the serum hepaplastin test and ICG retention rate similarly improved postoperatively. A follow-up superior mesenteric arteriogram was performed during the venous phase, demonstrating resolution of the shunt and decreased portal vein narrowing. The patient has suffered no further episodes of disturbed consciousness postoperatively.