Indwelling femoral venous catheters were prospectively studied by ultrasonography to define the frequency and evolution of inferior vena cava (IVC) thrombosis. IVC thrombosis was identified in six of 56 catheters (54 children). Only one patient with a positive ultrasound scan had clinical signs of thrombosis. All children with IVC thrombosis had had catheters in place for over six days. It is recommended that either the femoral central venous catheters are routinely changed at six days or ultrasound studies are routinely performed twice a week in all patients with catheters in situ for six or more days and that the catheter is removed immediately if evidence of thrombosis appears.