Three patients undergoing retrograde left ventricular catheterization experienced cerebral emboli with catastrophic neurologic consequences. In each instance, a Teflon-coated guide wire was used to facilitate passage through the aortic valve. This finding led to a detailed investigation of the procedures and materials used. The thrombogenicity of both Teflon and stainless steel guide wires was determined in 10 mongrel dogs. Clot formation was readily identified within a few minutes on all guide wires. In each instance clotting was significantly less on steel than on Teflon-coated wires (P <0.001). A benzalkonium chloride-heparin solution applied to the guide wires just before their introduction into the artery resulted in prolongation of the time required for clot formation. Further investigations should be performed with this solution. Although our experiments may not be directly applicable to man, we recommend that the increased thrombogenicity of Teflon-coated wires be considered, When such wires are needed, they should be used with caution and preferably only in the descending aorta for brief periods, or in conjunction with systemic hepatization. © 1973, Dun·Donnelley Publishing Corporation. All rights reserved. All rights reserved.