The feasibility and safety of percutaneous venous and arterial catheterization in infants and children was investigated prospectively from July 1, 1970 to July 1, 1971. Femoral vessel catheterization using an indwelling sheath was performed in 565 of 582 consecutive procedures attempted. The percutaneous technique was unsuccessful and a venous cutdown procedure required in 17 patients, a failure rate of 3 percent. Prior percutaneous procedures had been performed in 71 patients and, in most instances, the same vessels were used for the repeat catheterizations. Percutaneous retrograde arterial catheterization was attempted and accomplished in 260 patients. Left heart catheterization was achieved by passage of the venous catheter through a foramen ovale, atrial septal defect or ventricular septal defect in 229 of the 565 percutaneous venous prograde studies. A diminished arterial pulse was noted immediately after the procedure in 16 of the 260 patients who underwent retrograde arterial catheterization. Twelve of these 16 patients weighed less than 15 kg. The pulse remained diminished in only 3 of these 16 patients 3 to 6 months later. Advantages of percutaneous sheath cardiac catheterization include the ability to use the same vessel for repeat studies, the small incidence of arterial complications, the ease of making many changes in catheter as well as the ability to use any type of catheter. In addition, there is a greater chance of entering the left side of the heart with a prograde femoral venous catheter which, in many patients, may avoid performance of a retrograde arterial procedure.