Nine cases of hepatocellular carcinoma which have invaded the intra- and extrahepatic portal vein were evaluated by computed tomography (CT) and ultrasonography (US). The outstanding CT and sonographic features of the portal vein tumor thrombus were described. In CT, contrast opacification of normal portal vein and its major tributaries were not observed, and they were replaced by a soft tissue density mass representing the tumor thrombus. The thrombus measured 35 - 45 Hounsfield units (HU) in precontrast scans and 60 - 80 HU in postcontrast scans. The portal vein tumor thrombus showed a branched pattern of low density in porta hepatis and intrahepatic region due to differences in attenuation coefficients of the thrombus and adjacent noncancerous hepatic parenchyma on postcontrast scans. In ultrasound study, tumor thrombus was recognized as an echogenic solid mass in the porta hepatis obliterating the normal portal venous structures, or as an intraluminal solid mass in the dilated portal vein and its branches. Numerous collateral venous channels surrounding the thrombosed portal vein were also demonstrated on both CT and US. Particular emphasis was placed on the clinical implications of these non-invasive detection of the portal vein tumor thrombosis in patients with hepatocellular carcinoma.