The aim of this prospective study was to investigate the incidence of pulmonary embolism after pacemaker implantation, and to assess the benefits, if any, of postoperative prophylactic heparin therapy. Twenty of 40 patients were given low-dose heparin therapy; the remaining 20 patients were the control group and were not treated. Assignment to one of these groups was made alternately, in chronological order according to the time of implantation. Only tined leads with silicone insulation were used. All patients were mobilized on their first postoperative day. Pulmonary scintigraphy was performed before implantation, two weeks after implantation and again 12 months later. A pulmonary embolism was defined as evidence of new perfusion defects after implantation. There were no such defects in the group treated with heparin; the control group (20 patients) had three cases of perfusion defects within a 14-day postoperative period. Twelve months later, no further perfusion defects were observed in any patient. Thus, our study revealed a 15% incidence of asymptomatic pulmonary embolism following pacemaker implantation in patients not given prophylactic heparin therapy.