Patients with idiopathic portal hypertension (IPH) are known to have sclerotic changes of the intrahepatic portal vein radicles. In order to elucidate the pathological changes in the extrahepatic portal venous system in IPH, studies were carried out on the portal trunk in 12 patients with IPH, 59 patients with liver cirrhosis including some with associated hepatocellular carcinoma, and 12 normal matched control subjects. Histological examinations including histomorphometry were performed on the transverse sections of the portal trunk taken at autopsy. Most of the patients with IPH showed severe phlebosclerosis which was more pronounced than seen in liver cirrhosis. Thrombosis was also frequently observed in IPH. In IPH, the portal trunk was characterized by fibrous thickening of the intima and media with a prominent increase of elastic fibers. The mean area and thickness of the intima and media were significantly greater than in patients with liver cirrhosis. Sclerosis extensively involving both the extrahepatic and intrahepatic ramifications of the portal vein appeared to be characteristic of IPH.