In this issue of Pediatrics, Neal et al. report a 95% incidence of thrombosis with catheterization of an umbilical artery. Like the authors, I was surprised at this high rate of thrombus formation. It is, however, consistent with the experience of those who have studied the problem of deposition of thrombotic material on indwelling peripheral arterial catheters in children and adult patients. To some extent, the formation of a thrombus is dependent upon the composition of the catheter. Polyethylene, polyvinyl (from which the Argyle catheter is made), and polyurethane have rather rough surfaces and are quite thrombogenic. Nachmani et al. observed fibrin adhering to polyurethane catheters within 15 minutes of placement in an artery.