Idiopathic portal hypertension is reported in five cases including one case of chronic arsenical intake and one case of chronic industrial vinyl chloride exposure. In all five cases the patients presented with gastrointestinal bleeding as the chief complaint. Physical examination was within normal limits except for splenomegaly in all. Results of liver function tests were normal, except for the relative clearance of sulfobromophtalein. A surgical liver biopsy specimen was obtained in all cases and showed moderate degrees of portal fibrosis, but no cirrhosis. Combined umbilicoportal, hepatic vein and superior mesenteric artery catheterization was performed in all cases. Hepatoportographies showed distortion of the intrahepatic portal venous system and cut-off of small portal venules. Porto-hepatic gradients ranged from 14.0 to 20.5 mm Hg. The portal hypertension was both sinusoidal and presinusoidal in nature but mainly presinusoidal. Hepatic extraction of indocyanine green and of albumin microaggregates was normal, thereby suggesting normal functional portal blood supply to the liver. The patients with arsenical or vinyl chloride exposure could not be differentiated from the other three patients with idiopathic portal hypertension. These results suggest that idiopathic portal hypertension may be related to domestic or industrial exposure to other hepatotoxins.