[en] We aimed to prospectively and quantitatively assess the effects of aortic valve replacement (AVR) for aortic stenosis (AS) on mitral regurgitation (MR) and to examine the determinants of the changes in MR. Fifty-two patients with AS scheduled for AVR were included if holosystolic MR not being considered for replacement or repair was detected. MR was quantified using the proximal isovelocity surface area method before and 8 +/- 4 days after surgery. Mitral valvular deformation parameters did not change significantly, but the mitral effective regurgitant orifice (ERO) and regurgitant volume decreased from 11 +/- 6 mm(2) to 8 +/- 6 mm(2) and from 20 +/- 10 ml to 11 +/- 9 ml, respectively (both p <0.0001). Using multiple linear regression analysis, preoperative severity of MR, mitral leaflet coaptation height, and end-diastolic volume decrease were independently associated with postoperative reduction in MR, whereas changes in mitral valve morphology after surgery were not. MR etiology did not predict the reduction in MR. In conclusion, the decrease in MR observed in most patients after AVR is associated with the magnitude of acute left ventricular reverse remodeling. As the reduction in left ventricular systolic pressure contributes to the decrease in regurgitant volume, the preoperative quantitative assessment of MR should best be performed by measurement of the ERO.