Perforation of the inferior vena cava has been reported following insertion of the Kim-Ray Greenfield filter. This report describes this complication as a consequence of a preventable technical error. However, the clinical implications of such a malpositioned filter may extend beyond the possible obvious complications of retroperitoneal hemorrhage or paracaval organ injury. Inadequate protection against pulmonary embolism may result. A three-dimensional concept is presented to illustrate the loss of equal "compartmentalization" of the cava.