AIMS: The conceptual framework of proportionate versus disproportionate mitral regurgitation (MR) translates poorly to individual patients with heart failure (HF) and secondary MR. A novel index, the ratio of MR severity to left atrial volume (LAV), may identify patients with "disproportionate" MR and a higher risk of events. The objectives, therefore, were to investigate the prognostic impact of MR severity to LAV ratio on outcomes among HF patients with severe secondary MR randomized to transcatheter edge-to-edge repair (TEER) with the MitraClipTM device plus guideline-directed medical therapy (GDMT) vs. GDMT alone in the COAPT trial. METHODS AND RESULTS: The ratio of preprocedural regurgitant volume (RVol) to LAV was calculated from baseline transthoracic echocardiograms. The primary endpoint was 2-year covariate-adjusted rate of HF hospitalization (HFH).Among 567 patients, the median RVol/LAV was 0.67 (IQR 0.48-0.91). In patients randomized to GDMT alone, lower RVol/LAV was independently associated with an increased 2-year risk of HFH (adjHR: 1.77; 95% CI: 1.20-2.63). RVol/LAV was a stronger predictor of adverse outcomes than RVol or LAV alone. Treatment with TEER plus GDMT compared with GDMT alone was associated with lower 2-year rates of HFH both in patients with low and high RVol/LAV (Pinteraction = 0.28). Baseline RVol/LAV ratio was unrelated to 2-year mortality, health status, or functional capacity in either treatment group. CONCLUSIONS: Low RVol/LAV ratio was an independent predictor of 2-year HFH in HF patients with severe MR treated with GDMT alone in the COAPT trial. TEER improved outcomes regardless of baseline RVol/LAV ratio. CLINICAL TRIAL REGISTRATION: Trial Name: Cardiovascular Outcomes Assessment of the MitraClip Percutaneous Therapy for Heart Failure Patients With Functional Mitral Regurgitation (The COAPT Trial) (COAPT) ClinicalTrial.gov Identifier: NCT01626079 URL: https://clinicaltrials.gov/ct2/show/NCT01626079.