Seventeen patients with accepted M mode echocardiographic criteria for flail mitral leaflet were studied. M mode echocardiograms revealed characteristic disordered mitral valve motion: (1) 16 (94 percent) had chaotic diastolic mitral motion; (2) 14 (82 percent) had systolic mitral flutter; (3) 14 (82 percent) had systolic left atrial echoes; and (4) 12 (71 percent) had systolic mitral valve prolapse. In 8 patients (47 percent) all four findings were present, with three findings present in 16 (35 percent) and two findings present in 13 (18 percent); none had fewer than two findings. Cross-sectional echocardiographic studies in 10 patients revealed a systolic whipping motion of the posterior mitral leaflet into the left atrium in all, abnormal systolic mitral coaptation in all and an abnormal mass of systolic left atrial echoes in 4. None of the first three M mode criteria were observed in 230 patients with uncomplicated "mid systolic click-late systolic murmur" syndrome; cross-sectional echocardiography in 30 of 230 patients revealed normal systolic mitral coaptation and no systolic whipping of the tip of the posterior mitral leaflet into the left atrium.