We analyzed the association of left ventricular end-systolic diameter (LVESD) with survival after diagnosis in organic mitral regurgitation (MR) due to flail leaflets.LVESD is a marker of left ventricular function in patients with organic MR but its association to survival after diagnosis is unknown.The Mitral regurgitation International DAtabase (MIDA) is a multicenter registry of echocardiographically-diagnosed organic MR due to flail leaflets. We enrolled 739 patients with MR due to flail leaflets (65±12 years; ejection fraction [EF] 65±10%) in whom LVESD was measured (36±7mm).Under conservative management, 10-year survival and survival free of cardiac death were higher with LVESD <40mm vs. ≥40mm (64±5% vs. 48±10%; p<0.001, and 73±5% vs. 63±10%; p=0.001). LVESD ≥40mm independently predicted overall mortality (HR 1.95[1.01-3.83]) and cardiac mortality (HR 3.09[1.35-7.09]) under conservative management. Mortality risk increased linearly with LVESD above 40mm (HR 1.15[1.04-1.27] per 1mm increment). There was no interaction between presence of symptoms or EF<60% and LVESD impact on survival (p=0.69). During the entire follow-up (including post-surgical), LVESD ≥40mm independently predicted overall mortality (HR 1.86[1.24-2.80]) and cardiac mortality (HR 2.14[1.29-3.56]), due to persistence of excess mortality in patients with LVESD ≥40mm after surgery (HR 1.86[1.11-3.15] for overall death, and 1.81[1.05-3.54] for cardiac death).In MR due to flail leaflets, LVESD ≥40mm is independently associated with increased mortality under medical management, but also after mitral surgery. These findings support prompt surgical rescue in patients with LVESD ≥40mm but also suggest that best preservation of survival is achieved in patients operated before LVESD reaches 40mm.