BACKGROUND: Ischemic mitral regurgitation (MR) is classically ascribed to functional restriction of normal leaflets, but recent studies have suggested post-myocardial infarction (MI) mitral valve (MV) leaflet fibrosis and thickening, challenging valve normality. Progression of leaflet thickness post-MI has not been studied. We hypothesized that excessive MV remodeling post-MI contributes to MR. Our objectives are to characterize MV changes after MI and relate them to MR. METHODS AND RESULTS: Three groups of 40 patients with serial echocardiograms over a mean of 23.4 months were identified from an echocardiography database: patients first studied early (6±12 days) and late (12±7 years) after an inferior MI and normal controls. MV thickness was correlated with MR. We studied the mechanisms for MV changes in a sheep model (6 apical MI versus 6 controls) followed for 8 weeks, with MV cellular and histopathologic analyses. Early post-MI, leaflet thickness was found to be similar to controls (2.6±0.5 vs 2.5±0.4 mm; =0.23) but significantly increased over time (2.5±0.4 to 2.9±0.4 mm; <0.01). In this group, patients tolerating maximal doses of renin-angiotensin blocking agents had less thickening (25% of patients; <0.01). The late-MI group had increased thickness (3.2±0.5 vs 2.5±0.4 mm; <0.01) without progression. At follow-up, 48% of post-MI patients had more than mild MR. Increased thickness was independently associated with MR. Experimentally, 8 weeks post-MI, MVs were 2-fold thicker than controls, with increased collagen, profibrotic transforming growth factor-β, and endothelial-to-mesenchymal transformation, confirmed by flow cytometry. CONCLUSIONS: MV thickness increases post-MI and correlates with MR, suggesting an organic component to ischemic MR. MV fibrotic remodeling can indicate directions for future therapy.