Thirty patients with primary hepatocellular carcinoma were studied by computed tomography (CT) and angiography. Tumor images on CT could be classified into four groups according to the differences between precontrast and postcontrast scans. In comparing the findings of angiography and CT, hypervascular tumor shadows on angiography were visualized as low density areas on CT. However, in several cases, the vessel character of the tumor, hypervascularity, and prolonged capillary staining may have been responsible for the CT tumor appearance after contrast medium administration. The studies of cases with therapeutic embolization of the hepatic artery showed a marked decrease in density on CT. Patients with obstruction of branches of the portal vein had homogeneous low densities in the areas drained by the obstructed branches. Although the characteristics of the hepatic artery played an important role, portal blood flow was as significant in determining equally or more noticeably the CT appearance of hepatocellular carcinoma.