Bacground: Although adaptive change in mitral leaflet area (MLA) has recently been suggested as an important pathogenic mechanism in several diseases, few data are available regarding normal reference value of MLA and its determining factors. Methods: Real-time 3-dimensional echocardiography was done in 61 healthy volunteers (Age 51짹15 years [19-74]; M/F=28/33). Customized software was used to obtain a validated measure of MLA, mitral annular area, non-planarity and LV mass. Results: MLA showed individual variation with mean value of 10.0짹2.3 cm² (range, 5.2-14.5) and positive correlation with body surface area (BSA, R=0.61, p<0.001). Indexed MLA (MLA/BSA) was not different between male and female (6.3짹1.0 vs. 5.9짹1.3 cm ² /m² /, p=0.19). End-systolic volume(R=0.48, p<0.001) and end diastolic volume (R=0.57, p<0.001) was univariately associated with MLA. Left atrial dimension (R=0.54, p<0.001) and LV mass (R=0.82, p<0.001) were also univariate determinants of MLA. Stepwise multiple linear regression analysis revealed that LV mass was an independent determinant of MLA. Conclusion: Adjustment for BSA is needed for comparison of MLA. LV mass was independently associated with mitral leaflet area. Further study is necessary to evaluate a potential common mechanism of mitral leaflet elongation and LV hypertrophy.