SUMMARY The true incidence of thrombosis and pulmonary embolism in patients requiring temporary transvenous pacing via the femoral vein is unknown. Twenty-nine patients, mean age 66.2 years (range 38-79 years) who required temporary pacing were studied by bilateral contrast venography and perfusion lung scanning after removal of the pacing catheter. The patients were subdivided into two subgroups based on hemodynamic status. There was no statistical difference between the two groups with respect to mean age or the time the pacer catheter was in place and all patients were placed on low-dose heparin therapy. Ten patients (34%) had venographic evidence of thrombosis; six of them (60%) showed lung scan evidence of pulmonary emboli. The hemodynamically compromised group consisted of seven patients, six (85%) of whom had thrombosis; three of these six (50%) had pulmonary emboli. Of the 22 patients in the hemodynamically stable group, four (18%) had thrombosis and two of these (50%) had pulmonary emboli. Venous thrombosis, with subsequent pulmonary emboli, is a serious complication of temporary transvenous pacing using the femoral approach, despite the use of prophylactic low-dose heparin.TEMPORARY TRANSVENOUS endocardial pacing is a simple, safe and effective means of treating both high-grade atrioventricular block' and selected cardiac dysrhythmias.2 9 Several venous routes are available for the insertion of the pacing electrode. These include both cutdown and percutaneous techniques using either the brachial, subclavian, internal jugular or femoral veins.Recently the percutaneous transfemoral venous technique has been advocated4 6 as a simple method that is free of significant complications. Lumia and Rios6 reported that the overall complication rates for the brachial and femoral approaches were statistically similar, 9.1% and 7.8%, respectively. In their series, phlebitis was clinically evident in 18.8% of patients in whom the brachial vein was used, while the femoral approach did not produce clinically evident phlebitis. However, pulmonary emboli were clinically detected only in patients in whom the femoral vein was used. Weinstein et al.7 prospectively studied 100 consecutive patients who required temporary pacing and in whom the femoral vein was used. Thrombophlebitis and thromboembolism were not complications of the femoral route.This study was designed to prospectively analyze the incidence of femoral vein thrombosis, with subsequent pulmonary embolization, in patients who required temporary transvenous pacing using the percutaneous femoral technique.