Objectives: The valve-in-valve (VinV) procedure may be used in high-risk patients with failed mitral surgical bioprostheses. The objective of this in vitro study was to assess the hemodynamic function of different VinV configurations. Methods: A double activation duplicator was used to test 11 valve configurations (surgical bioprostheses alone) and 15 VinV configurations (Sapien [Edwards Lifesciences, Irvine, Calif] implanted within the surgical bioprosthesis) under 8 different hemodynamic conditions. The internal orifice diameter (IOD) of the surgical bioprosthesis was measured with a Smartscope (OGP Multi Sensor Measuring Instruments, Singapore). Results: The VinV procedure was associated with significant deterioration in antegrade hemodynamic parameters compared with valve configuration (effective orifice area, 1.51 +/- 0.21 cm 2 vs 1.65 +/- 0.37 cm 2; P <. 001 and regurgitant fraction, 11.5% +/- 7.2% vs 4.8% +/- 3.8%; P <. 001). Among the 120 tested experimental VinV situations, moderate or greater mitral stenosis occurred in 52 situations and mild or greater regurgitation occurred in 28 situations. The IOD of the surgical bioprosthesis was themain independent determinant of effective orifice area and regurgitant fraction. An IOD< 22 mm was associatedwith higher risk of significantmitral stenosis, particularly when the oversizing was > 20%, and IOD > 23 mm was associatedwith higher risk of paravalvular regurgitation when oversizing was <8%. Conclusions: This in vitro study shows that VinV within mitral surgical bioprostheses provides satisfactory hemodynamic results in the majority of patients. However, significantmitral stenosis is more likely to occur when the IOD of the surgical bioprosthesis is< 22 mm, and particularly when the percentage of oversizing is > 20%. Significant paravalvular regurgitation is rare and occurs with larger IODs and lower percentage of oversizing (8%).