Analysis of velocity acceleration proximal to a regurgitant valve has been proposed as a method to quantify the regurgitant flow rate (Qo). Previous work has assumed inviscid flow through an infinitesimal orifice, predicting hemispheric isovelocity shells, with calculated flow rate given by Qc=2πrN2vN, where vNis user-selected velocity of interest and rNis the distance from that velocity to the orifice. To validate this approach more rigorously and investigate the impact of finite orifice size on the assumption of hemispheric symmetry, numerical and in vitro modeling was used. Finite-difference modeling demonstrated hemispheric shape for contours more than two orifice diameters from the orifice. More proximal than this (where the measured velocity vNexceeded 3% of the orifice velocity vo), flow was progressively underestimated, with a proportional error ΔQ/Qonearly identical to the ratio of contour velocity to orifice velocity, vN/vo. For the in vitro investigations, flow rates from 4.3 to 150 cm3/sec through 0.3 and 1.0 cm2circular orifices were imaged with color Doppler with aliasing velocities from 19 to 36 cm/sec. Overall, the calculated flow (assuming hemispheric symmetry) correlated well with the true flow, Qc=0.88Qo-7.82 (r=0.945, SD=12.2 cm3/sec,p<0.0001,n=48), but progressively underestimated flow when the vNapproached the orifice velocity vo. Applying a correction factor predicted by the numerical modeling, ΔQ was improved from −13.81 ± 13.01 cm3/sec (mean ± SD) to +1.54 ± 5.67 cm3/sec. These data indicate that flow can be accurately calculated using the hemispheric assumption as Qc=2πrN2vNwhen vN<<vo. For larger vN, flow is systematically underestimated, but a more accurate estimate may be obtained by multiplying Qcby vo/(vo-vN). These observations lend additional support for the clinical use of the proximal acceleration concept and suggest a simple correction factor to make a more accurate estimation of valvular regurgitation. (Circulation Research1992;70:923–930)