Color-coded Doppler sonography (CCDS) was used for the examination of tumor thrombus (n = 7) and benign thrombosis (n = 6) of the inferior vena cava. Tumor thrombus was due to hepatocellular carcinoma in two cases and to renal cell carcinoma in five cases. Whereas no specific information about the nature of thrombus formation could be gained by gray scale sonography, a typical patchy vascularization pattern was noted within tumor thrombi in six of seven cases using CCDS. This ws due to marked neovascularization within the tumor thrombi, confirmed by histologic examination in all cases. In one patient no vascularization within the tumor thrombus could be observed by CCDS because the thrombus was relatively small. In patients with bland thrombosis, either no vascularization of the thrombus was seen (n = 5) or linear recanalization of the thrombus occurred (n = 1). Therefore, it might be possible to differentiate tumor from nontumor thrombus in the inferior vena cava by using CCDS. Further studies are needed to confirm this preliminary hypothesis.