The accuracy of transesophageal echocardiography was compared with that of transthoracic echocardiography in the detection of ruptured chordae tendineae (flail mitral leaflet) in 27 patients with mitral valve prolapse (MVP) who underwent valve repair or replacement for mitral regurgitation. Confirmation of the presence of ruptured chordae resulting in a flail leaflet was available at surgery in all cases. The echocardiographic studies were read blindly by 2 independent observers with any differences resolved by a third. Mean (+/- standard deviation) age was 63 +/- 13 years. Men (n = 20) outnumbered women (n = 7) (p less than 0.02), and tended to be younger (p = 0.06). Flail leaflets were identified in 20 of 27 patients. In 1 patient, both leaflets were involved and in the remaining 19 patients posterior leaflets (15 patients) were more frequently affected than anterior leaflets (4 patients). Transesophageal echocardiography correctly identified all 20 patients with flail leaflets, but 1 false positive study occurred among the 7 patients without a flail leaflet. In contrast, transthoracic echocardiography identified only 12 of 20 patients with flail leaflets, with no false positive studies. Transesophageal echocardiography was more accurate, correctly classifying 26 of 27 (96%) cases versus 19 of 27 (70%) by the transthoracic approach (p less than 0.01). This study suggests a higher incidence of chordal rupture to the posterior leaflet in patients with MVP and demonstrates improved accuracy of transesophageal over transthoracic echocardiography in the detection of flail leaflets.