The purpose of this study was to evaluate the possible role of CDFI in identifying malignant renal tumor thrombus in the renal vein or the IVC. This study involved 24 tumors in 23 patients, including 19 renal cell carcinomas, four Wilms' tumors, and one rhabdoid tumor. CDFI had an overall sensitivity of 95% in predicting combined renal vein and IVC tumor involvement but was more sensitive in the renal vein alone (100%) than in the IVC alone (89%). Specificity of CDFI was nearly equal for both renal vein and IVC thrombus at 85%. The main limitation of CDFI was its inability to predict venous tumor extension in large or bulky tumors. We would recommend CDFI as a method of detecting renal vein or IVC thrombus at the time of initial real-time sonographic detection of tumor. When CT or MR imaging is equivocal, CDFI may be used to predict tumor thrombus in the renal vein or IVC.