This study evaluated the safety, efficacy, and effect of MitraClip treatment on symptoms and left ventricular (LV) remodeling in nonresponders to cardiac resynchronization therapy (CRT).Moderate to severe functional mitral regurgitation (FMR) frequently persists after CRT, contributing to reduced or no response to CRT. Percutaneous repair with the MitraClip has been proposed as an additional therapeutic option in select patients with significant FMR.Fifty-one severely symptomatic CRT nonresponders with significant FMR (grade ≥2, 100%) underwent MitraClip treatment. Changes in New York Heart Association functional class, degree of FMR, LV ejection fraction (EF), and LV end-diastolic/end-systolic volumes (EDV/ESV) before and after (3, 6, and 12 months) MitraClip implantation were recorded. Mortality data, including cause of death, were collected.MC treatment was feasible in all patients (49% 1 clip, 46% 2 clips). There were 2 periprocedural deaths. Median follow-up was 14 months (25th to 75th percentile: 8 to 17 months). New York Heart Association functional class improved acutely at discharge (73%) and continued to improve progressively during follow-up (regression model, p < 0.001). The proportion of patients with significant residual FMR (grade ≥2) progressively decreased during follow-up (regression model, p < 0.001). Reverse LV remodeling and improved LVEF were detected at 6 months, with further improvement at 12 months (regression model, p = 0.001, p = 0.008, and p = 0.031 for ESV, EDV, and LVEF, respectively). Overall 30-day mortality was 4.2%. Overall mortality during follow-up was 19.9 per 100 person-years (95% confidence interval: 10.3 to 38.3). Nonsurvivors had more compromised clinical baseline conditions, longer QRS duration, and a more dilated heart.FMR treatment with the MitraClip in CRT nonresponders was feasible, safe, and demonstrated improved functional class, increased LVEF, and reduced ventricular volumes in about 70% of these study patients.