Objective To investigate the characteristics of minimal fat renal angiomyolipoma (MFAML)and clear cell renal cell carcinoma(CCRCC)in high resolution multi-slice spiral CT(MSCT)and to improve the diagnosis accuracy for the renal tumors.Methods A retrospective analysis was performed on 24 MFAML patients(16 females,8 males)with mean age of 43(19-74)years and 24 CCRCC patients(16 females,8 males)with mean age of 44(21-76)years.All patients had undergone MSCT and proved histopathologically after surgery.The characteristics included tumor location,tumor attenuation on unenhanced CT,enhancement characteristics(degree of tumor enhancement in the early corticomedullary phase,homogeneity of enhancement,amount of enhancement,enhancement pattern over time),tumor margin,intratumoral calcification,and perinephric changes.The predictive value of each CT characteristic was determined by using multivariate logistic regression analysis.Results The tumor location in the kidney (upper pole:MFAML,6 cases,CCRCC,6 cases;middle:MFAML,7 cases,CCRCC,9 cases;lower pole:MFAML,11 cases,CCRCC,9 cases)and smooth tumor margin(MFAML,n=21;CCRCC,n=19)were not significantly different between MFAML patients and those with CCRCC,P＞0.05.Twenty-one cases of both MFAMLs and CCRCCs had the significant enhancement in the early corticomedullary phase,which were hypovascular tumors,whereas the mean amount of tumor enhancement was greater in CCRCC than in MFAML in both the early corticomedullary and the corticomedullary phases(CCRCC:175 HU,196 HU;MFAML:125 HU,145 HU;P＜0.05.MFAML usually showed homogeneous enhancement(n=15)rather than heterogeneous enhancement(n =9),whereas most CCRCC showed heterogeneous enhancement(n =17)rather than homogeneous enhancement(n =7),P＜0.05).Enhancement pattern was not a significant predictor.Within the 13 MFAML cases,8 cases had sufficient blood supply(6 cases showed obvious wash-in-and-wash-out,2 cases were with prolonged enhancement),5 cases with hypovascular showed a pattern of prolonged or gradual enhancement,while 21 CCRCC cases had sufficient blood supply and 71％ of them showed obvious wash-in-and-wash-out.High tumor attenuation on unenhanced scans(MFAML:17 patients (75％);CCRCC:2 patients(8％),P=0.002,OR=0.010)and threshold enhancement values of 129.5 HU in the corticomedullary phase(MFAML:5 patients(20％);CCRCC:20 patients(83％),P =0.004,OR =0.057)were valuable predictors for differentiating MFAML from CCRCC at multivariate logistic regression analysis.Conclusions MSCT is useful in differentiating MFAML from CCRCC,with high tumor attenuation on unenhanced scans and threshold enhancement values of 129.5 HU in the corticomedullary phase being the most valuable CT findings.75％ of MFAMLs with sufficient blood supply also show a pattern of wash-in-and-wash-out,which can easily misdiagnosed as a renal cancer.