The influence of postoperative radiotherapy (RT) on the patency and blood flow of the internal jugular veins (IJV) after functional neck dissection (FND) was assessed in a prospective study. Doppler ultrasonography of the IJV was performed before and after surgery. Fifty-four FND were evaluated in 29 oncological patients who underwent surgery between September 1994 and February 1997. Twelve IJV received postoperative RT. There were no cases of thrombosis before or after surgery in any case. Blood-flow speed in the right Valsalva maneuver was the only parameter affected in irradiated patients.