AIMS: In the western world, idiopathic non-cirrhotic portal hypertension (INCPH) is a rare disease. This study aimed to investigate the histopathological features in western INCPH patients and to assess pathological differences between liver specimens of INCPH with and without HIV. METHODS AND RESULTS: Biopsies of 70 INCPH patients (of which 15 were HIV-infected) were compared to 23 patients with non-cirrhotic portal vein thrombosis (PVT), which served as a control group for non-cirrhotic portal hypertension. Phlebosclerosis, nodular regeneration (NR), sinusoidal dilatation, paraportal shunting vessels, perisinusoidal fibrosis and portal tract remnants were the most prevalent morphological features of INCPH. There were significant (P < 0.01) morphological differences between INCPH and PVT liver specimens with regard to portal tract remnants (46% versus 0%), phlebosclerosis (95% versus 65%), portal vein dilatation (34% versus 78%) and NR (56% versus 22%). The degree of NR correlated with the severity of phlebosclerosis (P < 0.01). NR was seen more frequently in the HIV-INCPH group, compared to the non-HIV-infected patients (P < 0.001). CONCLUSION: Portal tract remnants, phlebosclerosis and nodular regeneration are typical features of INCPH. Sinusoidal dilatation, paraportal shunting vessels and increased portal and parenchymal vessels might represent pressure-related morphological signs of portal hypertension. Finally, more nodular regeneration was observed in HIV-associated INCPH.