Purpose To evaluate the severity of aortic regurgitation (AR) quantitatively using MDCT. Methods and Materials 23 patients with AR underwent electrocardiography (ECG) gated 40-slice MDCT (Philips Brilliance 40) and transthoracic or transesophageal echocardiography. CT data sets were reconstructed during mid-systole, mid-diastole, end-systole, and end-diastole phases (35%, 45%, 75%, 95% of R-R interval). The abnormally opened aortic valve area in the end-diastole (AR area) was planimetrically measured on CT reformatted images. We compared AR area on CT (ARACT) with severity of AR (mild, moderate, or severe) estimated at echocardiography. The depth, width, intensity, and pressure half time of the aortic regurgitant jet in the parasternal and apical views were considered for echocardiographic grading of AR. Valve morphology on reformatted and volume-rendered CT images were also assessed. Five patients with severe AR at echocardiography underwent aortic valve replacement surgery. Results In 7 patients with mild AR, ARACT was 0.14 짹 0.04 cm짼(mean 짹 1SD; range, 0.09 ~ 0.21). In 8 patients with moderate AR, ARACT was 0.44 짹 0.19 cm짼(range, 0.26 ~ 0.81). In 8 patients with severe AR, ARACT was 1.1 짹 0.45 cm짼(range, 0.5 ~ 1.83). With a cutoff value of 0.70 cm짼for significant or severe AR, sensitivity, specificity, and diagnostic accuracy of MDCT as compared echocardiographic severity were 88%, 93%, and 95%, respectively. In 5 surgically treated patients, mean ARACT was 1.00 cm짼(range, 0.5 ~ 1.49). Conclusion Planimetric measurements of the AR area using MDCT is useful in quantitative evaluation of the severity of aortic regurgitation.