PURPOSE: Tumoral neovascularity characteristic of venous invasion of renal cell carcinoma has been demonstrated using angiography, computed tomography, and color Doppler duplex ultrasonography. Previous reports on the characteristic findings of MR imaging have, however, been sporadic. The purpose of this study was to evaluate the usefulness of a patchy signal seen on MR images within the venous thrombus of renal cell carcinoma as a sign of tumor neovascularity. MATERIALS AND METHODS: Six patients with histopathologically-proven unilateral renal cell carcinoma (right:left=3:3) with venous tumor thrombi were included in this study. MR imaging findings were retrospectively analyzed with respect to the presence of patchy signals within a thrombus in a renal vein, the IVC, or the right atrium. MR imaging findings on spin-echo and gradient-echo images were compared with histopathologic findings especially in terms of tumor neovascularity. RESULTS: On histopathologic examination, a tumor thrombus was found in the renal vein in three patients, in the IVC in six, and within the right atrium in two. Patchy signals were demonstrated on MR images in one of three patients with renal vein invasion, in four of six with IVC invasion, and in both patients with right atrial invasion. CONCLUSION: On MR imaging, a patchy signal within a venous thrombus is a helpful sign suggesting tumor neovascularity, one of the characteristics of venous invasion by renal cell carcinoma.