To test the hypothesis that mitral valve prolapse may be due either to billowing of mitral leaflets into the left atrium or to dynamic expansion of the mitral anulus, mitral leaflet and annular dimensions and motion were measured by computer-assisted two-dimensional echocardiography in 35 normal adults and 48 subjects with auscultatory and M-mode echocardiographic evidence of mitral prolapse. Among normal subjects, mitral leaflet and annular dimensions tended to be larger compared with body size or left ventricular size in women than in men. Mitral leaflet billowing was observed in 24 (50%) of 48 patients with mitral prolapse and 0 of 35 normal subjects (100% specificity). The 24 patients without leaflet billowing had greater systolic expansion of the mitral anulus (p less than 0.0001) than did normal adults or patients with leaflet billowing (41 +/- 14% versus 27 +/- 12% and 22 +/- 11%, respectively) and a significantly lower body mass index (p less than 0.005 versus normal group). The ratio of anterior plus posterior mitral leaflet length to end-systolic annular diameter was lower in patients with prolapse without leaflet billowing than in normal subjects (1.09 +/- 0.12 versus 1.19 +/- 0.15, respectively, p less than 0.01) or patients with leaflet billowing (1.21 +/- 0.17, p less than 0.05). Among 35 relatives with mitral prolapse in the families of 23 patients with prolapse, the pattern was the same as in the proband in 31 (89%) (p less than 0.000002).(ABSTRACT TRUNCATED AT 250 WORDS)