Among patients with chronic nonischemic mitral regurgitation (MR), high short-term mortality risk can be identified by left (LV) and/or right ventricular (RV) ejection fraction (EF) criteria (LVEF ≤45% and/or RVEF ≤30%). Mitral valve replacement or repair (MVR) significantly improves outcome in this subgroup, but predictors of late postoperative survival are not known, and the benefit of MVR has not been defined in patients matched for severity of LV and RV dysfunction. Therefore, prospective assessment of 14 consecutive high risk MR patients was performed before MVR and during 9 years (average) postoperatively to define echocardiographic and radionuclide angiographic predictors of survival; survival also was evaluated in a contemporaneous series of 9 high risk unoperated MR patients, and in subgroups of operated and unoperated patients matched for EF. Of 14 MVR patients, 4 died (3 cardiac: 1 sudden, 2 congestive heart failure). Only preoperative RVEF ≤20% significantly predicted postoperative deaths (rest p = 0.032; exercise p = 0.05). Of 9 unoperated patients, 8 died. Mortality risk of unoperated patients remained higher than that of MVR patients when groups were matched for preoperative LVEF (p = 0.0001). Among patients with RVEF >20%, MVR significantly improved survival versus medical treatment (rest: p < 0.0001, exercise: p = 0.0003). In high risk MR patients, MVR improves survival; preoperative RV performance can define subgroups with different long-term postoperative survival.