This report concerns two unusual complications of endocardial pacing: (1) simultaneous thrombosis of the superior and inferior venae cavae and (2) supravalvular, valvular, and infravalvular stenosis of the tricuspid valve. The management of both cases required removal of all endocardial leads with the aid of cardiopulmonary bypass, with inferior vena cava thrombectomy in the first case and tricuspid valve replacement in the second. From our review of the literature, we have reached the following conclusions: (1) Clinical thrombosis secondary to endocardial pacing leads is rare (1.2%), but venography in asymptomatic patients revealed venous thrombosis in 44% of the patients studied. (2) Endocardial pacing leads should have the smallest outer diameter possible, preferably with an electro-negative, thromboresistant surface, and should be implanted with minimal redundancy. The risk of venous thrombosis should be kept in mind when recommending atrioventricular sequential pacing systems requiring multiple leads passing through the superior vena cava. (3) Retained leads which are infected or have migrated and become redundant or looped are at risk of causing septicemia, thrombosis, or septic embolization and should be removed. The high incidence of subclinical venous thrombosis suggests that reasonable efforts be made to remove all retained leads.