Our objective is to develop a new transthoracic Doppler echocardiographic technique to determine mitral regurgitant fraction. The standard color Doppler method for assessment of mitral regurgitation is semiquantitative and dependent on instrument gain. By using the mitral and aortic valve continuous wave Doppler velocities, one can determine regurgitant fraction. This technique takes into account the flow dependence of the mitral valve area. Two constants, A and B, which represent the flow dependence of the mitral valve area and the ratio of the mitral valve area to aortic valve area at zero flow, respectively, were determined by regression in 36 patients without valvular disease (r = .89). Thirty patients with isolated mitral regurgitation were then studied. The mitral regurgitant fraction was calculated from the following: Regurgitant fraction = 1 - TVIav/Bf[Vmv/(1 - AVmv)]dt, where TVIav is the time velocity integral across the aortic valve, Vmv is the continuous wave velocity across the mitral valve, and A and B are constants. The regurgitant fraction was then compared with color Doppler assessment of mitral regurgitation assessed by independent observers. In patients with mitral regurgitation, there was a strong correlation between standard visual assessment and our new Doppler method (Kendall's tau b rank correlation = 0.65; p < .001). The new Doppler method was able to correctly categorize 90% of patients with mild mitral regurgitation and 88% of patients with severe mitral regurgitation; however, there was poorer agreement with the color Doppler assessment of moderate mitral regurgitation. Mitral regurgitant fraction can be calculated with our new Doppler method. This method is quantitative, objective, nongain dependent, and separates mild from severe mitral regurgitation well.