A total of 47 preoperative CT scans in patients with renal cell carcinoma were retrospectively reviewed and compared with surgical findings to assess the accuracy of CT for determining the perinephric tumor extension. CT criteria for perinephric tumor extension. CT criteria for perinephric extension were hazy ill-defined tumor margin perirenal nodule and fascial thickening Regardless of the tumor stage, the accuracy of the accuracy of CT in detecting perinephric extension was 76.6% (36/47), croscopic infiltration of the perinephric space. The causes of overstaging were tumor infiltration to the renal capsule (n=5), partial adhesion with the perinephric fat (n=3) and renal vein thrombosis (n=1) A smooth clear tumor margin is highly reliable sign for stage I but infiltrative findings onto renal capsule and perirenal fat could be considered stage I.