The purpose of this study was to evaluate the M-mode, two-dimensional, and Doppler echocardiographic signs for a flail mitral valve leaflet. This was a retrospective evaluation of 54 patients who had (1) significant mitral regurgitation, (2) a technically adequate echocardiographic study, and (3) description of valve anatomy done at surgery or necropsy. The following M-mode signs were examined for their ability to detect a flail valve: (1) systolic flutter of the mitral valve closure line, sensitivity 29%, specificity 76%; (2) abnormal diastolic posterior leaflet motion, sensitivity 73%, specificity 71%; (3) abnormal diastolic anterior leaflet motion, sensitivity 67%, specificity 86%; (4) systolic atrial echoes, sensitivity 28%, specificity 68%; (5) multiple independent systolic closure lines, sensitivity 71%, specificity 52%. The two-dimensional echocardiographic signs evaluated were (1) diastolic inversion of the anterior leaflet toward the left atrium, sensitivity 29%, specificity 96%; (2) diastolic inversion of the posterior leaflet toward the left atrium, sensitivity 54%, specificity 93%, (3) systolic inversion of the anterior leaflet into the left atrium, sensitivity 57%, specificity 93%; (4) systolic inversion of the posterior leaflet into the left atrium, sensitivity 79%, specificity 86%; (5) systolic whipping of the mitral leaflets, sensitivity 73%, specificity 74%; (6) presence of floating apical chordae, sensitivity 30%, specificity 91%. Doppler echocardiographic signs evaluated were (1) presence of left atrial systolic antegrade flow, sensitivity 30%, specificity 91%; (2) vertical striations superimposed on the typical regurgitant flow pattern, sensitivity 75%, specificity 69%. When all the two-dimensional signs except systolic whipping and the M-mode signs for abnormal diastolic leaflet motion were combined, the sensitivity for detecting a flail mitral valve was maximized at 97%, but specificity was reduced to 64%. In conclusion, two-dimensional echocardiographic signs are more sensitive and specific than either M-mode or Doppler signs for detecting a flail mitral valve. The various M-mode, two-dimensional, and Doppler echocardiographic signs, however, are complementary to each other, and sensitivity is maximized when they are combined.