Background: Diagnosing severe aortic stenosis (AS) depends on flow and pressure conditions. It is suspected that concomitant aortic regurgitation (AR) has an impact on the assessment of AS severity. The aim of this study was to analyze the impact of concomitant AR on Doppler-derived guideline criteria. We hypothesized that both transvalvular flow velocity (maxVAV) and the mean pressure gradient (mPGAV) will be affected by AR, whereas the effective orifice area (EOA) and the ratio between maximum velocity of the left ventricular outflow tract and transvalvular flow velocity (maxVLVOT/maxVAV) will not. Furthermore, we hypothesized that EOA (by continuity equation), and the geometric orifice area (GOA) (by planimetry using 3D transesophageal echocardiography, TEE), will not be affected by AR. Methods and Results: In this retrospective study, 335 patients (mean age 75.9 ± 9.8 years, 44% male) with severe AS (defined by EOA < 1.0 cm2) who underwent a transthoracic and transesophageal echocardiography were analyzed. Patients with a reduced left ventricular ejection fraction (LVEF < 53%) were excluded (n= 97). The remaining 238 patients were divided into four subgroups depending on AR severity, and they were assessed using pressure half time (PHT) method: no, trace, mild (PHT 500–750 ms), and moderate AR (PHT 250–500 ms). maxVAV, mPGAVand maxVLVOT/maxVAVwere assessed in all subgroups. Among the four subgroups (no (n= 101), trace (n= 49), mild (n= 61) and moderate AR (n= 27)), no differences were obtained for EOA (no AR: 0.75 cm2± 0.15; trace AR: 0.74 cm2± 0.14; mild AR: 0.75 cm2± 0.14; moderate AR: 0.75 cm2± 0.15,p= 0.998) and GOA (no AR: 0.78 cm2± 0.20; trace AR: 0.79 cm2± 0.15; mild AR: 0.82 cm2± 0.19; moderate AR: 0.83 cm2± 0.14,p= 0.424). In severe AS with moderate AR, compared with patients without AR, maxVAV(p= 0.005) and mPGAV(p= 0.022) were higher, whereas EOA (p= 0.998) and maxVLVOT/maxVAV(p= 0.243) did not differ. The EOA was smaller than the GOA in AS patients with trace (0.74 cm2± 0.14 vs. 0.79 cm2± 0.15,p= 0.024), mild (0.75 cm2± 0.14 vs. 0.82 cm2± 0.19,p= 0.021), and moderate AR (0.75 cm2± 0.15 vs. 0.83 cm2± 0.14,p= 0.024). In 40 (17%) patients with severe AS, according to an EOA < 1.0 cm2, the GOA was ≥ 1.0 cm2. Conclusion: In severe AS with moderate AR, the maxVAVand mPGAVare significantly affected by AR, whereas the EOA and maxVLVOT/maxVAVare not. These results highlight the potential risk of overestimating AS severity in combined aortic valve disease by only assessing transvalvular flow velocity and the mean pressure gradient. Furthermore, in cases of borderline EOA, of approximately 1.0 cm2, AS severity should be verified by determining the GOA.