To investigate the effect of external radiotherapy in the control of portal vein invasion (PVI) in hepatocellular carcinoma (HCC), ten patients with cytologically confirmed unresectable HCC were recruited for study. All of the patients were assigned Pugh's classification A, and all had only unilateral PV involvement. The main tumors were treated by transcatheter arterial embolization. The PVI was irradiated with a dose of 3000-5000 cGY using a linear accelerator under localization by real-time ultrasound. All ten patients responded to the external irradiation, with complete disappearance of the PVI occurring in five and partial shrinkage, in the other five. However, the HCC extended to the contralateral PV in two patients, although the irradiated lesion had shrunk. Both patients had shown involvement of the main PV in the initial study. Six patients died after 3, 6, 7, 7, 8, and 10 months, respectively, due to advance of the HCC, rupture, liver failure, and respiratory failure. The others survived for longer than 6 months and remain under follow-up. The branch of PVI at discovery might have an important bearing on the effect of the radiotherapy. No postirradiation hepatitis or other complication was observed.