A titanium Greenfield inferior vena cava filter was used for the treatment of 2 patients with unresectable renal cell carcinomas with tumor thrombi to prevent a fatal pulmonary embolism induced by tumor clots released during systemic interferon therapy and embolization of the primary tumor. After treatment, the size of the renal cell carcinomas at the primary site and the tumor thrombi decreased by 50%. There were no fatal pulmonary embolisms or complications related to the filter during the observation period (24 and 25 months) after therapy. This method may be useful in the prevention of a fatal pulmonary embolism induced by embolization and systemic interferon therapy in these patients.