Quantitative assessment of the severity of mitral regurgitation (MR) is based on the calculation of the effective regurgitant orifice (ERO), a measure of lesion severity, and of the regurgitant volume (RVol), a measure of left ventricular volume overload. We aimed at evaluating the determinants of RVol in both organic (OMR) and functional mitral regurgitation (FMR).MR severity was quantitatively assessed using the proximal isovelocity surface area (PISA) method in 240 patients, 142 with OMR and 98 patients with FMR. By definition, ERO and RVol were strongly correlated both in patients with OMR and FMR (both R = 0.90, P 40 mmHg, P < 0.0001) but not in patients with normal SPAP (≤40 mmHg, P = 0.09). In multivariate analysis, independent determinants of RVol were ERO (P < 0.0001), MR mechanism (FMR/OMR) (P = 0.0003) and SPAP (P = 0.03). In patients with elevated SPAP, ERO (P < 0.0001), left ventricular ejection fraction (LVEF) (P = 0.03), and MR mechanism (P = 0.03) were independently associated with RVol, whereas in patients with normal SPAP, ERO (P < 0.0001) was the only independent determinant of RVol.In the present study, we evaluated the contrasting effect of similar lesion severity in OMR and FMR and showed that similar ERO were associated with lower RVol in FMR compared with OMR. The regurgitant volume is the result of complex interactions of anatomic lesions, LVEF, and SPAP and our results highlight the importance of taking into account these parameters when interpreting RVol values in clinical practice, especially in FMR.