The hepatic perfusion index (HPI), the ratio of hepatic arterial to total liver blood flow, was determined by radionuclide angiography in 28 subjects with normal livers and 62 patients with portal hypertension of various aetiologies. The latter group comprised 50 patients with cirrhosis (14 Child class A, 20 Child class B, 16 Child class C) and 12 patients with non-cirrhotic portal hypertension (7 non-cirrhotic portal fibrosis, 5 extrahepatic portal venous obstruction). The mean (+/- S.D.) HPI was significantly higher among the patients with cirrhosis (Child class A, 53.9 +/- 18.1; Child class B, 65.6 +/- 29.4; Child class C, 78.6 +/- 33.5) and non-cirrhotic portal hypertension (54.9 +/- 17.7) compared with the subjects with normal livers (35.6 +/- 10.5). The patients with non-cirrhotic portal hypertension had a mean value similar to that of the cirrhotic patients in Child class A. A higher HPI was associated with worsening liver status. We conclude, therefore, that the HPI will be high in portal hypertension irrespective of aetiology and a rise in the HPI may indicate a deterioration in the condition of the liver.