Regurgitant orifice area (ROA) measurements by cardiac CT have not been compared with a quantitative classification of aortic regurgitation (AR) severity based on regurgitant fraction by phase-contrast cardiovascular magnetic resonance (PC-CMR).To compare ROA using dual-source CT (DSCT) with the grade of AR using PC-CMR for obtaining the cutoff values of the ROA allowing for grading of AR severity.We retrospectively enrolled 208 patients (81 women, mean age 53.3 ± 14.4 years) with AR who underwent DSCT and CMR. DSCT data sets were reconstructed in 10% steps from 0% to 90% of the R-R interval to measure ROA. Grades of AR were determined by regurgitant fraction using PC-CMR. Receiver operating characteristic curves were calculated to differentiate between grades of AR and ROA.Sixty-three patients with mild AR, 80 with moderate AR, and 65 with severe AR by PC-CMR were enrolled. Quantification of the ROA by DSCT (mean, 27 ± 21 mm 2 ) was significantly correlated with the grade of AR by PC-CMR (r = 0.83). In the receiver operating characteristic analysis, discrimination (mild vs moderate and moderate vs severe) among grades of AR with DSCT was accurate when cutoff ROAs of 15 mm 2 and 23 mm 2 in comparison with PC-CMR were used.The cutoff values of the ROA by DSCT allow for grading of AR severity determined by PC-CMR but are lower than those previously published, using transthoracic echocardiography as the reference standard. A multicenter study is necessary with a wide range of population for obtaining cutoff ROAs.