Introduction: Surgical repair of functional mitral regurgitation (FMR) with undersizing mitral annuloplasty (UMA) is a common practice. Though excessive annular downsizing can improve coaptation, it raises the risk of functional mitral stenosis (FMS). Predicting if certain patients can develop FMS after UMA, can help with surgical planning and inform the safe levels of downsizing. In this study, we sought to investigate that the pre-operative diastolic geometry of the mitral valve can predict the risk of developing FMS with UMA. Methods: Pig mitral valves (n=8) were excised and mounted into a left heart simulator with steady diastolic flow only [ Fig1A ]. Three leaflet tethering patterns were mimicked in this model: annular dilation (AD), AD + asymmetric tethering (Asym) and AD + symmetric tethering (Sym) [ Fig1B ]. Under each condition, the annulus was downsized to different levels by an adjustable UMA [ Fig1C ]. Millar pressure catheter and Echocardiography were used to measure MV pressure gradient and diastolic configuration under various conditions. Results: Under each tethering geometry (AD, AD + Asym and AD + Sym respectively), pressure gradient gradually increases as the downsizing level increases. Also, at each specific level of downsizing, pressure gradient increases as the subannular tethering increases. Similarly, at each specific level of downsizing, EOA decreases as the subannular tethering increases. Under each tethering geometry, EOA also decreases as the downsizing level increases [ Fig1D ]. In addition, it is demonstrated that at different flow rates, both pressure gradient and EOA correlates with anterior diastolic angle, which is impacted mainly by preoperative subannular tethering [ Fig1E ]. Conclusions: In FMR, the hemodynamic outcome of MV after UMA depends on both the extent of annular downsizing and preoperative leaflet tethering geometry, both of which should be taken into account when deciding the optimal surgical intervention.