idespread use of ultrasonography and abdominal CT has increased the detection of small renal neoplasms, often found incidentally in asymptomatic patients. 1,2 RCC and AML are the most frequently occurring malignant and benign renal parenchymal tumors, respectively. Frequently small RCC are hyperechoic 1-3 and may mimic AML. Efforts were made to differentiate the two conditions on the basis of ultrasonographic characteristics such as intratumoral cysts, hypoechoic rim, and posterior acoustic shadowing. [3][4][5][6] These findings were fairly specific but not sensitive enough. 6,7 In our experience as well as that of others, AML tend to be more echogenic than RCC. 3,7,[8][9][10] Some investigators tried to differentiate these two tumors using a visual assessment of echogenicity alone without satisfactory results. 3,6 The purposes of this study were to evaluate the echogenicity of renal masses objectively and quantitatively by using CATEQ technique and to assess the value of this technique in differentiating small hyperechoic RCC from AML. To assess the value of computer-aided tissue echo quantification in differentiating small hyperechoic renal cell carcinomas from angiomyolipomas, we studied ultrasonographic images of 15 renal cell carcinomas and 20 angiomyolipomas. After digitizing the images, we measured the absolute gray scale values of the renal cortex, central echo complex, and mass. The relative gray scale value (%) of the mass was calculated by setting the gray scale value of the cortex as 0% and the central echo complex as 100%. The relative gray scale value of renal cell carcinomas was in the range of 12 to 73% (mean, 28%), whereas that of angiomyolipomas was 30 to 204% (mean, 130%). The differentiation between small hyperechoic renal cell carcinomas and angiomyolipomas can be facilitated by computer-aided tissue echo quantification.