Objective: We studied whether differences exist between hearts having rheumatic mitral valves and those having myxomatous mitral valves, in functional, geometrical, and mass changes in the left ventricle after mitral valve replacement.Methods: Patients who underwent mitral valve replacement without preservation of annular-papillary continuity for pure mitral incompetence were classified into rheumatic and myxomatous based on valvular histopathology. Echographic data measured before surgery was compared to that about 3 weeks after surgery.Results: In the rheumatic group, ejection fraction decreased from 57.5±10.8 to 47.7±12.0, indexed left ventricular internal dimension in systole (mm/m2) from 24.7±6.8 to 20.7±6.2 (P = 0.0001), and left ventricular mass index (g/m2) from 205±55 to 138±54 (P = 0.0002). In the myxomatous group, ejection fraction decreased from 60.4±11.6 to 39.7±14.5 (P = 0.0001), indexed left ventricular internal dimension in systole from 24.2±5.6 to 23.1±5.5, and left ventricular mass index from 195±83 to 111± 72 (P = 0.0004). Mean wall thickness index and relative wall thickness showed significant differences between the two groups postoperatively but no significant difference preoperatively.Conclusion: Hearts dilated due to chronic mitral incompetence respond differently after valvular replacement with total chordal excision depending on whether a rheumatic or myxomatous mitral valve is involved.