The aim of this study is to prospectively validate a recently reported semiquantitative index of mitral regurgitation (MR) severity. MR is a common echocardiographic finding with no single reference standard to evaluate its severity. We recently developed and retrospectively tested a semiquantitative index of MR severity. The MR index is a composite of 6 echocardiographic variables: jet penetration, proximal isovelocity surface area, continuous-wave Doppler characteristics of the regurgitant jet, pulmonary artery pressure, pulmonary venous flow pattern, and left atrial size. Sixty-two consecutive patients with varying grades of MR were prospectively studied. Patients were divided into 3 groups for comparison: mild MR, moderate MR, and severe MR. Each patient was evaluated for the 6 variables, with each variable scored on a 4-point scale (0 to 3). The reference standards for MR severity were qualitative evaluation by an expert, measurement of the regurgitant fraction (RF), and the effective regurgitant orifice area. The MR index increased in proportion to MR severity with a significant difference among the 3 groups (F = 84; p <0.0001). The MR index also correlated with RF (r = 0.73; p <0.0001) and the effective regurgitant orifice area (r = 0.74; p = 0.0001). A MR index ≥2.2 identified 13 of 16 patients with severe MR (sensitivity 82%, specificity 98%, positive predictive value 93%). No patient with severe MR had a score <2.0 and no patient with mild MR had a score >1.67. These results concurred with those obtained in a previously published retrospective study. Thus, the MR index is a simple, reproducible semiquantitative estimate of MR severity, that is widely applicable in routine clinical practice.