To test the value of acceleration flow signals proximal to the leaking orifice in assessing the severity of prosthetic mitral valve regurgitation, 39 consecutive patients undergoing left ventriculography were examined by Doppler color flow imaging. Acceleration flow signals proximal to the regurgitant orifice were detected in 27 of the 31 patients who had prosthetic mitral regurgitation by left ventriculography (sensitivity 87%). All four patients without acceleration flow signals had mild prosthetic mitral regurgitation by angiography. No acceleration flow signals were detected in any patient without prosthetic regurgitation by left ventriculography (specificity 100%). Individual values of the maximal area of acceleration flow signals obtained from three orthogonal planes in seven patients with mild prosthetic mitral regurgitation by angiography ranged from 0 to 17 mm2 (mean 4 +/- 6). In 8 patients with moderate prosthetic mitral regurgitation by angiography, the maximal area of acceleration flow signals ranged from 21 to 58 mm2 (mean 33 +/- 15), whereas the maximal area of acceleration flow signals in 16 patients with severe prosthetic regurgitation ranged from 20 to 173 mm2 (mean 102 +/- 41). The maximal area of the acceleration flow signals from three planes correlated well with the angiographic grade of prosthetic mitral regurgitation. There was a significant difference in the maximal area of acceleration flow signals between mild and moderate (p less than 0.001), moderate and severe (p less than 0.001) and mild and severe (p less than 0.001) prosthetic mitral regurgitation. Thus, measurement of acceleration flow signals by Doppler color flow imaging is useful in assessing the severity of prosthetic mitral regurgitation.