Although the method of percutaneous catheterization described by Seldinger is a quick and safe tech-nic for introducing a catheter into the vascular system, the catheter must have a hole in the end to accommodate a guide wire. Closed-end catheters with side-holes are superior to open-end catheters for angiocardiography because they are less likely to recoil out of position during the injection, greater flow rates can be obtained by increasing the pressure of injection, and perforation of the myocardium during the injection is much less likely. Special closed-end catheters with solid flexible tips are especially valuable for crossing the aortic valve. These catheters cannot be passed over a guide wire as required by conventional percutaneous technics because they have no end-hole. Side-hole, dosed-end catheters are superior for aortography because the contrast medium can be directed into the vessel with less filling of extraneous vessels. Extraneous vessels often obscure the artery being studied. In order to extend the range of the percutaneous technic, the authors have developed a method for inserting any type of catheter into a vessel percuta-neously. Biopsy forceps, special closed-end catheters, or even lumenless catheters can all be inserted with ease and safety. A Mylar2 sheath with a wall thickness of 0.002 in., 18 em long (outside diameter of 0.084 and inside diameter of 0.080) fits snugly over a tapered end-hole Gensini Teflon3 catheter (Fig. 1). The outside diameter of a No. 6 French Gensini catheter is 0.079 in. The outside diameter of the Teflon catheter and sheath is 0.084 in., an increase of only 0.005 in. The vessel is punctured in the usual way, and the Teflon catheter is passed over a guide wire into the femoral or axillary artery for a distance of 10 cm. The guide wire is removed, and the Mylar sheath is advanced to the skin. The Teflon catheter and sheath together are advanced until 3 cm of the sheath is in the artery. At this point the Teflon catheter is withdrawn. The protruding end of the sheath is crimped with the fingers to stop the flow of blood. The closed-end catheter is inserted into the sheath and well into the vessel. The sheath is withdrawn to the hub of the catheter where it remains out of the way and can be used again if a further exchange of catheters is desired during the catheterization (Fig. 2). Sheath catheter systems can be made up in various sizes. A closed-end catheter of the same size as the inserting catheter or smaller can be used. To date we have used sheaths that will accommodate a No. 6 and a No. 7 French catheter. Ultrathin Teflon tubing is now commercially available but has a wall thickness of 0.008 in. This would increase the overall diameter of the sheath to 0.016 in., or over one French size increase in the diameter of the system if a Teflon sheath were used. This system would require a much larger hole in the artery to insert the same size catheter as the Mylar sheath, which of course would be undesirable.