Background— To assess the prevalence, determinants, and prognosis value of right ventricular (RV) ejection fraction (EF) impairment in organic mitral regurgitation. Methods and Results— Two hundred eight patients (62±12 years, 138 males) with chronic organic mitral regurgitation referred to surgery underwent an echocardiography and biventricular radionuclide angiography with regional function assessment. Mean RV EF was 40.4±10.2%, ranging from 10% to 65%. RV EF was severely impaired (≤35%) in 63 patients (30%), and biventricular impairment (left ventricular EF<60% and RV EF≤35%) was found in 34 patients (16%). Pathophysiologic correlates of RV EF were left ventricular septal function (β=0.42, P <0.0001), left ventricular end-diastolic diameter index (β=−0.22, P =0.002), and pulmonary artery systolic pressure (β=−0.14, P =0.047). Mitral effective regurgitant orifice size (n=84) influenced RV EF (β=−0.28, P =0.012). In 68 patients examined after surgery, RV EF increased strongly (27.5±4.3–37.9±7.3, P <0.0001) in patients with depressed RV EF, whereas it did not change in others ( P =0.91). RV EF ≤35% impaired 10-year cardiovascular survival (71.6±8.4% versus 89.8±3.7%, P =0.037). Biventricular impairment dramatically reduced 10-year cardiovascular survival (51.9±15.3% versus 90.3±3.2%, P <0.0001; hazard ratio, 5.2; P <0.0001) even after adjustment for known predictors (hazard ratio, 4.6; P =0.004). Biventricular impairment reduced also 10-year overall survival (34.8±13.0% versus 72.6±4.5%, P =0.003; hazard ratio, 2.5; P =0.005) even after adjustment for known predictors ( P =0.048). Conclusions— In patients with organic mitral regurgitation referred to surgery, RV function impairment is frequent (30%) and depends weakly on pulmonary artery systolic pressure but mainly on left ventricular remodeling and septal function. RV function is a predictor of postoperative cardiovascular survival, whereas biventricular impairment is a powerful predictor of both cardiovascular and overall survival.