Patients with liver disease often have impairment of metabolic liver functions. The liver has numerous functions, which are essential for maintenance of life e.g. urea synthesis. This function is central in whole body nitrogen (N) homeostasis and removes ammonia, which is produced during N conversion and is extremely toxic especially for brain functions. Impairment of urea synthesis causes increased ammonia concentrations in the blood, which is taken up by the brain and contributes to development of hepatic encephalopathy in patients with liver disease. This condition is one of the most debilitating complications of liver disease as it affects cognitive functions and with increasing severity also behaviour and motor function. The mild stage (termed minimal hepatic encephalopathy) causes subtle cognitive dysfunction, which can only be detected by means of special tests. The more severe stages (manifest hepatic encephalopathy) are accompanied by manifest dysfunction of sleep pattern and orientation eventually leading to liver coma. The condition is reversible and no structural brain damage occurs.