Exact delineation of the local extension of renal cell carcinoma is essential for correct preoperative staging, planning of surgical approach, and prognostication. Recently, there have appeared many articles on therole of CT in staging renal cell carcinoma. However, little information is available regarding the accuracy of CTin determining the spread of renal cell carcinoma to adjacent muscles. Authors retrospectively reviewed andanalyzed CT scans of 34 patients wtih confirmed renal cell carcinoma to determine the accuracy of the diagnosis ofrenal cell carcinoma invasion of the perirenal muscles. 1. Obliteration of the fat planes between renal tumor andadjacent muscles or the diaphragmatic crura was noted in 22 patients(64.7%). There was, however, no actualneoplastic invasion of adjacent muscles, except in one patients who had nonspecific inflammation. 2. In the 22paitents with obliterated fat planes, nine showed ill-defined, irregular interface. Two of these nine patients hadan obtuse angle between renal mass and adjacent muscles without evidence of invasion. 3. Our study indicates thatthe obliteration of the fat plane is not reliable sign of muscle invasion.