Blood flow was measured with a mercury-in-rubber strain-gauge plethysmograph in the utilized and intact extremity during and 24 hours after uncomplicated (with no clinical signs of vascular insufficiency) percutaneous right and left cardiac catheterization in 20 children. In this group, there was a significant decrease of the flow in the utilized extremity after entry into the artery, with the flow returning to normal within 24 hours after the procedure. Flow was markedly reduced in the utilized extremity of a child who developed thrombosis of the femoral artery after catheterization. Seven additional children were studied one to four years after retrograde catheterization. In this group the flows in utilized and intact extremities were equal at rest and with reactive hyperemia. It is concluded that uncomplicated transfemoral percutaneous cardiac catheterization in children does not impair the blood flow in the limbs.