A total of 155 patients with hepatocellular carcinoma were studied by celiac and superior mesenteric angiography. Complete (9 patients) or near complete (1 patient) obstruction of the portal vein and formation of hepatopetal collateral veins in the porta hepatis, or the so-called cavernous transformation of the portal vein, were seen in 10 patients. In 4 patients, the first angiogram did not show cavernous transformation, but on the follow-up angiograms cavernous transformation was present. The suggested interval between obstruction of the portal vein and formation of cavernous transformation was no more than 5 wk. The mechanism of cavernous transformation and its clinical implications are briefly discussed.