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Background There is little evidence about the prognostic role of mitral regurgitation (MR) in patients with low-flow, low-gradient aortic stenosis undergoing transcatheter aortic valve replacement (TAVR). The aim of this study was to assess the prevalence and outcome implications of MR severity in patients with low-flow, low-gradient aortic stenosis undergoing TAVR, and to evaluate whether MR improvement after TAVR could influence clinical outcome. Methods and Results This study included consecutive patients with low-flow, low-gradient aortic stenosis undergoing TAVR at 2 Italian high-volume centers. The study population was categorized according to the baseline MR severity and to the presence of MR improvement at discharge. The primary outcome was the composite of all-cause death and hospitalization for worsening heart failure up to 1 year. The study included 268 patients; 57 (21%) patients showed MR >2+. Patients with MR >2+ showed a lower 1-year survival free from the primary outcome ( P <0.001), all-cause death ( P <0.001), and heart failure hospitalization ( P <0.001) compared with patients with MR ≤2+. At multivariable analysis, baseline MR >2+ was an independent predictor of the primary outcome ( P <0.001). Among patients with baseline MR >2+, MR improvement was reported in 24 (44%) cases after TAVR. The persistence of MR was associated with a significantly reduced survival free from the primary outcome, all-cause death, and heart failure hospitalization up to 1 year. Conclusions In this study, the presence of moderately severe to severe MR in patients with low-flow, low-gradient aortic stenosis undergoing TAVR portends a worse clinical outcome at 1 year. TAVR may improve MR severity in nearly half of the patients, resulting in a potential outcome benefit after discharge.