This prospective study extending for more than 3 years had two objectives: (a) to use Doppler ultrasonography (US) to estimate the incidence of asymptomatic catheter-related upper extremity deep venous thrombosis (DVT) in a large population and (b) to study the effect of the catheter position as an individual risk factor for catheter-related DVT. Between October 1995 and June 1998, a total of 145 patients who had oropharyngeal tract cancer and who were fitted with the same totally implantable central venous catheters (CVCs) were included in the study. Follow-up included (a) estimation of the position of each catheter tip on a chest radiograph obtained immediately after surgery and (b) regular monthly Doppler US screening for catheter-related DVT. Seventeen patients developed catheter-related DVT; 13 of them were asymptomatic. The mean interval between CVC implantation and detection of thrombosis was 42.2 days. Correct positioning of the distal catheter tip was associated with a significantly lower rate of catheter-related DVT. Only five of 87 patients with a correctly positioned distal catheter tip (ie, either in the superior vena cava or at the junction between the right atrium and the superior vena cava) developed thrombosis, compared with 12 of 26 patients with a misplaced catheter (P <.001). The side on which the CVC was implanted did not influence the catheter-related DVT rate. The rate of asymptomatic catheter-related DVT is high and could be lowered with correct initial CVC positioning.