A modified Seldinger percutaneous approach for catheterization was used for cardiac evaluation and monitoring of infants weighing less than 4,520 gm. From January 1968, through April 1971, 160 left- and 230 right-sided cardiac procedures were done. After July 1969 81% of all left-and 42% of all right-sided procedures were done percutaneously. After October 1969 catheters were also inserted percutaneously in 20 infants for intensive care monitoring. During this period three children were noted to have dimished pulse on discharge, two were noted to have vascular thrombosis, and two had episodes of bleeding from the catheter site requiring transfusion of fresh blood. Both instances of vascular thrombosis occurred in infants with monitoring catheters. The percutaneous method of catheter insertion into the femoral vessels offers a safe method for cardiac catheterization of the neonate and of insertion of monitoring catheters where the umbilical route is not feasible. Vessel continuity is also preserved, providing a route for repeated studies if necessary.