The molar ratio valine + leucine + isoleucine/phenylalanine + tyrosine was determined in the plasma of patients with liver disease of varying aetiology and severity and in an age and sex matched control group. In the control group of 58 subjects the mean ratio was 3.3 +/- 0.5 (ISD). The mean ratio was significantly lowered in groups of 25 patients with alcoholic cirrhosis (P less than 0.001), 25 patients with chronic active hepatitis (P less than 0.001), 23 patients with primary biliary cirrhosis (P less than 0.001), and 11 patients with cryptogenic cirrhosis (P less than 0.001). In a group of 50 patients with cirrhosis, the ratio was significantly lowered (P less than 0.001) irrespective of the presence of hepatic encephalopathy. A good correlation existed between the value of the ratio and the severity of the liver disease as judged histologically, with values of the ratio appearing to reflect histological change irrespective of the patient's clinical condition. There was no significant diurnal variation in the value of the ratio. Lowering of this plasma amino acid ratio appears to be secondary to liver disease and quite independent of the presence of hepatic encephalopathy.