Superior vena cava syndrome due to transvenous pacing leads is a rare event. We describe four cases. One occurred among 3,100 primary pacemaker insertions performed at our institution. In the other three cases the primary insertion had been performed elsewhere. Over 30 cases have been reported previously. Local infection, which preceded the development of superior vena cava syndrome in each of our four cases, and the presence of a severed retained lead, as in three of our cases, are important predisposing factors. There is no strong evidence that multiple lead insertion, if each lead remains intact, significantly increases the risk. The pathology at the site of obstruction includes thrombosis and in some cases fibrotic narrowing. Venous angiography is useful to show the site of obstruction, the extent of collateral circulation and to assess the response to treatment. Treatment should include removal of any infected pacemaker apparatus, anticoagulation and, if symptoms are of recent onset, thrombolytic therapy. Most patients improve but in those who do not angioplasty or surgical relief of the obstruction may be helpful.