A 56 year-old-male liver cirrhosis patient was admitted because of hepatic encephalopathy with hyperammonemia. We measured cerebral blood flow (CBF), cerebral oxygen extraction fraction (OEF) and cerebral metabolic rate for oxygen (CMRO2) by using PET before and after treatment of hyperammonemia. At the time of the first PET, serum ammonia was 152 micrograms/dl and on the second PET it was 88 micrograms/dl. The electroencephalogram and number connection test also improved on the second PET. CBF and CMRO2 were decreased by 25-42% in comparison to normal control and OEF increased 1-24% at first examination. Similar abnormal reduction of both CBF and CMRO2 and increase of OEF were seen at second examination. Reduced CBF and CMRO2 regardless of serum ammonia level suggests that heptic encephalopathy may be, at least in part, associated with pathological changes in brain tissue induced by hepatic metabolic disorder.