Background: The difference between renal oncocytomas (RO) and renal clear cell carcinomas (RCCs) presents the greatest diagnostic challenge. The aim of this study was to retrospectively determine if RO and RCCs could be differentiated on computed tomography (CT) images on the basis of their enhancement patterns with a new enhancement correcting method. Methods: Forty-six patients with a solitary renal mass who underwent total or partial nephrectomy were included in this study. Fourteen of those were RO and 32 were RCCs. All patients were examined with contrast-enhanced CT. The pattern and degree of enhancement were evaluated. We selected the area that demonstrated the greatest degree of enhancement of the renal lesion in the corticomedullary nephrographic and excretory phase images. Regions of interest (ROI) were also placed in adjacent normal renal cortex for normalization. We used the values of the normal renal cortex that were measured at the same time as divisors. The ratios of lesion-to-renal cortex enhancement were calculated for all three phases. The Student's t-test and Pearson's Chi-square test were used for statistical analyses. Results: All RCCs masses showed contrast that appeared to be better enhanced than RO on all contrast-enhanced phases of CT imaging, but there was no significant difference in absolute attenuation values between these two diseases (P > 0.05). The ratio of lesion-to-cortex attenuation in the corticomedullary phase showed significantly different values between RO and RCCs. The degree of contrast enhancement in RCCs was equal to or greater than that of the normal renal cortex, but it was less than that of the normal cortex in RO in the corticomedullary phase. The ratio of lesion-to-cortex attenuation in the corticomedullary phase was higher than the cut off value of 1.0 in most RCCs (84%, 27/32) and lower than 1.0 in most RO (93%, 13/14) (P < 0.05). In the nephrographic phase, the ratio of lesion-to-cortex attenuation was higher than that in the corticomedullary phase in most RO (71%, 10/14), showing a prolonged enhancement pattern; and was lower than that in most RCCs (97%, 31/32), showing an early washout pattern (P < 0.05). In the differentiation of RO from RCCs, the sensitivity was 93%, specificity 84%, positive predictive value 72%, negative predictive value 84%, and accuracy for RO was 87, if the ratio of lesion-to-cortex attenuation in a cortex phase was lower than the cutoff value of 1.0. The sensitivity was 71%, specificity was 97%, positive predictive value was 91%, negative predictive value was 91%, and accuracy for RO was 89%, if the ratio of lesion-to-cortex attenuation in nephrographic phase was higher than that in the corticomedullary phase. Conclusions: The ratios of renal lesion-to-cortex attenuation ratios may be helpful in differentiating RO from RCCs.