PURPOSE: The differentiation between renal cell carcinoma(RCC) and transitional cell carcinoma(TCC) is important due to the different methods of treatment and prognosis. But occasionally it is difficult to draw a distinction between the two diseases when renal parenchyme and renal collecting systems are invaded simultaneously. MATERIAL AND METHOD: We reviewed CT scans of 37 cases of renal cell carcinoma and 12 cases of transitional cell carcinoma which showed involvement of renal parenchyma and renal sinus fat on CT. Retrospective analysis was performed by 3 abdominal radiologists. Check points were renalcontour bulging or reniform shape, location of mass center, intact parenchyme overlying the tumor, cystic change, calcification, LN metastasis, vessel invasion, and perirenal extention. RESULT:There were renal contour bulging due to the tumor mass in 33 out of 37 cases of renal cellcarcinima, wherea and nine of 12 cases of transitional cell carcinoma maintained the reniform appearance. This is significant statiscal difference between the two(p<0.005). Center of all TCCs were located in the renal sinus, and 24 out of 35 cases of RCC were located in the cortex(p<0.005). Thirty-six out of 37 cases of RCC lost the overlying parenchyma, whereas 4 out of 9 cases of well enhanced TCC had intact overlying parenchyma(p<0.005) RCC showed uptic change within the tumor mags in 31 cases which was significanity higher than the 4 cases in TCC(p<0.05). CONCLUSION: CT findings of renal cell carcinoma are contour bulging, peripheral location, obliteration of parenchyma, and cystic change. Findings of transitional cell carcinoma are reniform appearance, central location within the kidney, intact overlying parenchyma, and rare cystic change.