Sixteen patients with hepatocellular carcinoma had duplex pulsed Doppler scanning of the portal thrombus and comparative angiography. Fifteen patients demonstrated detectable Doppler signals from the thrombus. The Doppler signals were corroborated by the "thread-and-streaks" sign on angiography in eight patients. This suggests that signals come from tumor vessels within the thrombus. Therefore, the presence of Doppler signals in the thrombotic portal venous system is not only due to partial obstruction of the lumen. Tumor thrombus from hepatocellular carcinoma must be considered, especially when the detectable signals are pulsatile waves.