To determine the causes of ruptured chordae tendineae and a suspected etiologic role for mitral valve prolapse (MVP), the mitral valve in 25 consecutive and surgically proved cases of chordal rupture were examined. The diagnosis of MVP was made on the basis of redundancy and marked hooding of the mitral leaflets and on histologic changes. MVP was the underlying morphologic abnormality in 23 patients, only 1 of whom had infective endocarditis that was responsible for the rupture. Thus MVP was the only underlying morphologic abnormality in 22 of 25 patients (88%). Another finding in this study was the demonstration of auscultatory and angiographic or echocardiographic evidence of MVP in 4 patients, aged 4 to 11 years (mean 7), before chordal rupture; no patient had had endocarditis. The morphologic and historical evidence would indicate that MVP is probably the most common cause of so-called spontaneous chordal rupture.