Non-cirrhotic, long-standing portal hypertension of unknown aetiology is being re-evaluated histopathologically and clinically. In this study, we examined 107 livers with this condition (92 wedge biopsy and 15 autopsy specimens) from five institutions in Japan. These cases were histologically categorized into four groups: idiopathic portal hypertension (66 cases), nodular regenerative hyperplasia (14 cases), partial nodular transformation (two cases), and incomplete septal cirrhosis (25 cases). These four groups shared several histological features: dense portal fibrosis with portal venous obliteration and intralobular slender fibrosis. In addition, the histopathological features characteristic of one group were also found to a mild degree in other groups. The histopathological lesions preceding portal venous obliteration remain speculative. However, the portal venous obliteration may be responsible for the occurrence of sustained portal hypertension and several of the pathological changes in these livers. It seems likely that idiopathic portal hypertension, nodular regenerative hyperplasia, partial nodular transformation and incomplete septal cirrhosis comprise a family of non-cirrhotic, long-standing portal hypertension in Japan, and the histological differences between them may reflect chronological progression of a single disease.