Congenital anomalies of the inferior vena cava (IVC) are not unusual and may complicate the placement of caval interruption devices. A duplicated IVC was demonstrated by venography in a patient with left iliofemoral venous thrombosis. A Greenfield filter was placed in each cavum from a right internal jugular vein approach. Because anomalies of the IVC are common, ascending venograms should be obtained in all patients who undergo caval interruption to document the number and position of the IVC, the location of the caval bifurcation, and the diameter of the IVC, as well as to identify the presence of thrombus within the lumen of the IVC. Unrecognized anomalies of the IVC may contribute to the incidence of recurrent pulmonary embolus after Greenfield filter placement.