Recent studies have shown suboptimal results of the proximal isovelocity surface area (PISA) method and the American Society of Echocardiography (ASE) algorithm for diagnosing severe primary mitral regurgitation (MR). The aim of this study was to evaluate the accuracy of regurgitant volume (RegVol) calculated using volumetric transthoracic echocardiography (TTE) for diagnosing severe primary MR.A total of 74 patients with primary MR due to prolapse or flail leaflet were prospectively recruited for both TTE and cardiac magnetic resonance (CMR) imaging. RegVol was calculated using PISA (RegVol_PISA) or the volumetric method (left ventricular total stroke volume - systolic forward outflow volume; RegVol_TTE). According to the ASE algorithm, patients with four or more parameters were diagnosed with severe MR. RegVol_CMR ≥ 60 mL was used as the gold standard for diagnosing severe MR.All subjects had at least moderate to severe MR according to ASE guidelines. CMR imaging confirmed that 30 patients (41%) had severe MR. The concordance correlation coefficient between RegVol_TTE and RegVol_CMR (0.809; 95% CI, 0.715-0.893) was higher than that between RegVol_PISA and RegVol_CMR (0.468; 95% CI, 0.323-0.576). The overall accuracy of RegVol_TTE for the diagnosis of severe MR was 90.5% (95% CI, 81.5-96.1), which was significantly higher than that of RegVol_PISA (64.9%; 95% CI, 52.9-75.6; P < .001) and the ASE algorithm (77.0%; 95% CI, 65.8-86.0; P = .004). The area under the curve for RegVol_TTE (0.95; 95% CI, 0.90-1.00) was significantly larger than that for RegVol_PISA (0.88; 95% CI, 0.80-0.96; P = .028).RegVol_TTE showed better diagnostic performance than the PISA method and the ASE algorithm in diagnosing severe MR. Further investigations are necessary to evaluate the clinical usefulness of the routine use of RegVol_TTE.