In order to assess prospectively the incidence and significance of venous thrombosis early after permanent transvenous pacemaker implantation venographic studies were carried out in 40 consecutive patients. The venograms performed between 1 and 6 months (mean 4 months) after the implantation were normal in 31 patients (77%), in six patients (15%) they showed partial venous obstruction and in three patients (8%) total obstruction. Between 6 and 12 months (mean 9 months) the venograms of five patients, that were previously normal, showed partial venous thrombosis. No changes were found in the venograms performed later. Only two of 14 patients with thrombosis of the great veins was clinically symptomatic and developed arm edema, that resolved spontaneously within about a month. No difference in incidence of abnormal venograms was found according to the type of insulation, the polarity of the electrode and the route of entry.