The growth rate of tumor thrombus in the portal vein and the concomitant enlargement of the volumes of the left lateral segment of the liver and the spleen were retrospectively investigated in 11 patients with hepatocellular carcinoma (HCC). In 7 of the 11 patients the left lateral segment of the liver enlarged as the tumor thrombus extended to the right second order branches and the more proximal order branches to the porta hepatis and eventually obstructed these branches. The volume of the left lateral segment was 1.3 to 2.7 times larger, compared with its original volume at the time of HCC detection. An increase in splenic volume was also found in 8 of the 11 patients. The increasing ratios of the splenic volume amounted to 1.3 to 2.1 times the volume found at the time of HCC detection. The growth of tumor thrombus in the right first order branch and portal trunk and the ensuing obstruction of these branches by the tumor thrombus resulted in the development of intractable ascites in 5 of the 11 patients and the development of hepatofugal collateral vessels in all 11 patients. These findings were not demonstrated prior to obstruction of these branches.