Twenty one patients with retained endocardial pacemaker leads were followed during a total observation period of 1097 months to assess the incidence of complications. Two patients developed thrombosis and occlusion of the superior vena cava, which was relieved by the development of a collateral venous circulation. In one patient the broken tip of the lead migrated to a pulmonary artery but did not cause overt complications. The remaining patients were free of symptoms. One patient died for reasons unconnected with pacemaker treatment. The good toleration of retained pacemaker leads by most patients indicates that major surgical procedures to remove the lead should be reserved for patients with life threatening complications, such as persistent infection or dangerous migration of the lead or both.