Implantable cardiac pacing leads must possess good thromboresistance. Three types of tubular insulator surfaces over the conductor wires were investigated. Materials tested were silicone rubber, smooth and roughened extruded polyurethane. Acute canine intra-arterial implantation methods were used. An index of thrombogenicity based on the weight of total thrombus formed per unit surface area of the vascular implant was determined for each test sample. Scanning electron microscope photographs of the lead surfaces were made. While silicone rubber leads were most thromboresistant, the polyurethane leads performed equally well.