Exact delineation of the local extension of renal cell carcinoma is essential for correct preoperative staging planning of surgical approach and pronostication. Recently there have appeared many articles on the role of CT in staging renal cell caricno-ma. However little information is availabe regarding the accuracy of Ct in determining the spread of renal cell carcinoma to adjacent muscles. Authors retrospectively reviewed and analyzed CT sacns of 34 patientw with confirmen renal cell carcinoma to determine the acuracy of the diagnosis of renal cell carcinoma invasion of the perirenal muscles. 1.Obiteration of the fat planes between renal tumor and adjacent muscles or the diaphragmatic crura was noted in 22 patients(64, 7%) There was however no actual neoplastic invasion of adjacent muscles except in one patient who had nonspecific inflammation. 2. in the 22 patients with obliterated fat planes none showed ill-defined irregular interface. two of these nine patients had an obtuse angle between renal mass and adjacent muscles without evidence of invasion. 3. Our study indicates that the obliteration of the fat plane is not reliable sign of muscle invasion.