Posterior displacement of the mitral valve with billowing into the left atrium has been the major echocardiographic criterion used for the diagnosis of mitral valve prolapse (MVP). However, the current criteria are limited by the influence of hemodynamic factors on the degree of prolapse, whereas complications such as mitral regurgitation, endocarditis, and need for surgery have been associated with redundancy or thickening of the leaflets. Sixty-eight normal subjects (mean age, 40 years; range, 18 to 76 years) were compared with 58 patients with MVP (mean age, 37 years, range, 18 to 83 years). Leaflet displacement across the annular plane in the parasternal long-axis view was mandatory for the diagnosis of MVP. Transthoracic echocardiographic measurements of anterior and posterior leaflet thickness, leaflet length, and chordal length were made from the parasternal long-axis view and the mitral annular diameter, from the apical four-chamber and two-chamber views. The MVP group had greater anterior thickness (4.1 +/- 0.4 mm vs 5.3 +/- 0.7 mm; p = 0.0001), posterior thickness (3.2 +/- 0.4 mm vs 4.7 +/- 0.9 mm; p = 0.0001), anterior length (22.8 +/- 2.0 mm vs 25.7 +/- 1.7 mm; p = 0.0001), posterior length (12.8 +/- 1.0 mm vs 15.7 +/- 2.5 mm; p = 0.0001), chordal length (25.6 +/- 2.7 mm vs 28.0 +/- 2.5 mm; p = 0.0001), and annular diameter (29.1 +/- 1.5 mm vs 31.3 +/- 2.6 mm; p = 0.0001). Of the MVP group, >80% had at least one abnormality identified and >50% had at least two abnormalities. In addition, patients with MVP with significant regurgitation had greater anterior thickness (5.2 +/- 0.7 mm vs 5.8 +/- 0.8 mm; p = 0.015), posterior thickness (4.5 +/- 0.9 mm vs 5.3 +/- 0.7 mm; p = 0.024), posterior length (15.1 +/- 1.6 mm vs 17.9 +/- 4.2 mm; p = 0.004), and annular diameter (36.0 +/- 2.0 mm vs 33.3 +/- 2.1 mm; p = 0.0001). The majority of patients with floppy mitral valves resulting in MVP have structural abnormalities that may be defined by echocardiography. A spectrum of floppy valve structure is demonstrated by echocardiography, with mitral regurgitation occurring more frequently in patients with multiple and more severe anatomic abnormalities. In addition to the presence of prolapse and regurgitation, the assessment of leaflet thickness, leaflet length, annular diameter, and chordal length is fundamental to the definition and stratification of patients with MVP associated with the floppy mitral valve.