Analysis of 25 liver biopsy specimens and one autopsy specimen from 26 Japanese patients (23 women and three men) with idiopathic portal hypertension revealed findings that collectively appeared diagnostic for the condition. Changes in the portal tract included capillary dilatation, phlebosclerosis, and fibro-elastosis of the stroma. Many portal veins were dilated and had herniated into the surrounding hepatic parenchyma. Portal vein obliteration and loss of bile ducts were a rare complication. The acinar architecture was disturbed by: 1) capillary and necro-inflammatory bridging, mostly between portal tracts and terminal hepatic veins; 2) the formation of isolated megasinusoids in a random distribution; 3) displaced and abnormally large hepatic vein branches with or without phlebosclerosis and 4) slender, curved fibrous septa (hairline septa). Early nodular regeneration was found in 25% of the cases. Our review supports the contention that incomplete septal cirrhosis may be a late manifestation of idiopathic portal hypertension. It is not clear whether the biopsy findings in Japanese patients differ only in severity from those in western patients, or whether the conditions differ pathogenetically. Some histopathological findings in the Japanese cases, in particular the necro-inflammatory changes, are difficult to reconcile with portal hypertension as a primary haemodynamic abnormality.