The present study clarified the clinical significance of the suction signal--regurgitant signal near the mitral valve in the left ventricle--as obtained by two-dimensional color Doppler echocardiography in patients with mitral regurgitation. The study population consisted of 39 patients with various heart diseases having mitral regurgitation. The presence of a mitral regurgitant signal was determined not only in the left atrium but in the left ventricle using the long-axis view, four-chamber view, and short-axis view. 1. The suction signal was observed in 17 cases; on the posteromedial commissure side in four; the central portion in seven; the anterolateral commissure side in five and the mitral aneurysm portion in one. The site of the suction signal indicated the mitral regurgitant orifice and was useful for identifying the regurgitant orifice particularly in cases with prolapse of the commissural scallop. 2. Left ventriculography was performed in 18 cases and the severity of mitral regurgitation was grade III/IV or IV/IV in seven of eight cases with suction signals and was grade II/IV in the remaining one, while it was grade II/IV or I/IV in all ten cases without the signal. Sensitivity was then 100% and specificity was 91% in diagnosing III/IV or greater regurgitation according to the presence of the suction signal. The left atrial jet distance, width and area in the 17 cases with suction signals were significantly greater than those of 22 cases without the signal (3.3 +/- 1.0 vs 1.9 +/- 0.7 cm, 1.4 +/- 0.7 vs 0.8 +/- 0.4 cm, and 4.2 +/- 2.8 vs 1.3 +/- 1.3 cm2, respectively). Thus, the suction signal in the left ventricle in mitral regurgitation provided useful information concerning not only the identification of a regurgitant orifice, but the grade of mitral regurgitation, as well.