Abstract Aims The concept of proportionate/disproportionate functional mitral regurgitation (FMR) has been limited by the lack of a simple way to assess it and by the paucity of data showing its prognostic superiority. The aim of this study was to evaluate the prognostic value of an individualized method of assessing FMR proportionality. Methods and results We retrospectively identified 572 patients with at least mild FMR and reduced left ventricular ejection fraction (<50%) under medical therapy. To determine FMR proportionality status, we used an approach where a simple equation determined the individualized theoretical regurgitant volume (or effective regurgitant orifice area) threshold associated with haemodynamically significant FMR. Then, we compared the measured with the theoretical value to categorize the population into non-severe, proportionate, and disproportionate FMR. The primary endpoint was all-cause mortality. During a median follow-up of 3.8 years (interquartile range: 1.8–6.2), 254 patients died. The unadjusted mortality incidence per 100 persons-year rose as the degree of FMR disproportionality worsened. On multivariable analysis, disproportionate FMR remained independently associated with all-cause mortality [adjusted hazard ratio: 1.785; 95% confidence interval (CI): 1.249–2.550; P = 0.001]. The FMR proportionality concept showed greater discriminative power (C-statistic 0.639; 95% CI: 0.597–0.680) than the American (C-statistic 0.583; 95% CI: 0.546–0.621; P for comparison <0.001) and European guidelines (C-statistic 0.584; 95% CI: 0.547–0.620; P for comparison <0.001). When added to any of the before-mentioned guidelines, FMR proportionality also improved risk stratification by reclassifying patients into lower and higher risk subsets. Conclusion Disproportionate FMR is independently associated with all-cause mortality and improves the risk stratification of current guidelines.