Increase in hepatic arterial flow in response to reduced portal flow (hepatic arterial buffer response) has been demonstrated experimentally and surgically. We provide pathologic evidence for hepatic arterial buffer response in non-cirrhotic patients with extrahepatic portal vein thrombosis and elucidate the histopathologic spectrum of non-cirrhotic portal vein thrombosis. Liver biopsies and resections from non-cirrhotic patients with extra-hepatic portal vein thrombosis were retrieved. Morphologic features, extent of CD34 staining, outer diameters, luminal diameters and wall thickness of hepatic arteries cut in cross-section and outer diameters of cross-sectioned paired bile ducts were compared with age- and gender-matched controls. There were 12 male and 9 female patients. Measurements of 280 and 193 arteries from patients and controls, respectively, demonstrated statistically significant (P<0.05) arterial dilatation (increase in percentage of arterial lumen to outer diameter) and arterial wall thinning in resection specimens of non-cirrhotic patients with extra-hepatic portal vein thrombosis. Subtle and/or focal dilatation of central veins, portal veins and sinusoids; focal trabecular thinning/thickening and mild ductular reaction were common findings in both the patient and control groups. Diffuse and obvious changes, and portal vein absence or attenuation were seen only in the patient group. Capillarization of sinusoids was not seen on CD34 stain. Two patients showed significant ductular reaction, one of who developed biliary strictures on follow-up. Hepatic arterial dilatation and wall thinning in non-cirrhotic patients with portal vein thrombosis provide pathologic evidence of hepatic arterial buffer response in the human liver. Obvious and diffuse sinusoidal dilatation and absence or attenuation of portal veins are highly suggestive of extrahepatic portal vein thrombosis in non-cirrhotic patients with portal hypertension. Periportal shunt vessels, hypervascular portal tracts, muscularized portal veins, large thick-walled or dilated arteries aid diagnosis but are rare findings. Normal or near-normal biopsies do not rule out portal vein thrombosis.