Using pulsed Doppler echocardiography, the blood flow dynamics in the left ventricular cavity were studied in 52 cases with mitral regurgitation (MR) (32 cases of grade 1 or 2 and 20 cases of grade 3 or 4 according to the Sellers' classification) were studied to quantify the severity of MR. Twelve healthy subjects served as the control. The results were as follows: At the mitral orifice, systolic laminar flow toward the left atrial cavity was observed in 17 cases with grade 3 or 4 MR. This flow was shown to begin at the isovolumic contraction phase and it had a higher velocity in end-systole. Thus, it could easily be differentiated from ejection flow observed in the left ventricular outflow tract. The duration of systolic blood flow in the mid-ventricle was significantly prolonged over that in the outflow tract in cases with grade 3 or 4 MR compared to those with grade 1 or 2 MR (grade 1 or 2: 4 +/- 27 msec, grade 3 or 4: 65 +/- 35 msec, p less than 0.001), indicating that MR continued even after the end of left ventricular ejection. The velocity of early diastolic left ventricular inflow was estimated for cases having isolated MR (16 cases with mitral chordal rupture and one case of mitral valve prolapse syndrome). In the healthy subjects and cases with grade 1 or 2 MR, the velocity was less than 84 cm/sec. The velocity was increased more than 100 cm/sec in grade 3 or 4 MR. These observations indicated the clinical potential of abnormal blood flow dynamics in the left ventricular cavity in the semi-quantification of MR.