Purpose: Adaptive increase of mitral leaflet area (MLA) has been recently reported in a model of ischemic mitral regurgitation (IMR). We aimed to elucidate geometric parameters associated with leaflet adaptation and factors determining MLA which are not well known. Methods: MLA was measured at mid-systole (closure area, CMLA) and diastole (total area, TMLA) using real-time 3-D echocardiographic data in normal controls (n=12) and patients with wall motion abnormalities (WMAs) in inferior or posterior wall (n=23, left ventricular [LV] ejection fraction 41짹9%). Tenting volume was also measured at mid-systole. Results: TMLA was increased by 39짹26 % in patients with WMAs compared with normal controls. TMLA showed positive correlation with LV mass (r=0.682, p<0.001). CMLA had positive correlation with tenting volume (r=0.827, p<0.001). Of 23 patients with WMAs, 12 patients showed significant IMR defined as proximal isovelocity surface area radius ���4 mm at the aliasing velocity of 40 cm/s. Although TMLA was not different between groups (18.8짹5.0 vs 18.2짹3.6 cm2, p=0.976), patients with significant IMR were characterized by older age (71짹7 vs 62짹11 years, p=0.009), larger CMLA (12.3짹4.3 vs 9.1짹1.6 cm2, p=0.016), smaller ratio of TMLA to CMLA (1.6짹0.3 vs 2.0짹0.5, p=0.011), and lager tenting volume (3.9짹2.1 vs 2.4짹0.8 cm3, p=0.043). Multivariate analysis showed that the ratio of TMLA to CMLA was the only determinant of significant IMR (odds ratio = 0.004, 95% confidence interval = 0.001-0.514, p=0.026). Conclusions: In vivo measurement of MLAs made it possible to appreciate both adequacy of leaflet adaptation and tethering geometry. TMLA shows parallel increase with change of LV mass, whereas CMLA represents tethering. Similar TMLA increase despite larger tenting volume in patients with ischemic MR suggests that inadequate leaflet adaptation is the main mechanism of IMR.