For half a century the systolic click and late systolic murmur lay dormant as innocent auscultatory curiosities. The thirteen years since Barlow related these phenomena to mitral leaflet prolapse have witnessed an astonishing information explosion. We have sought to bring together the accumulated data in this review. An Historical Perspective traces the evolution from the now abandoned "pericardial" or "extracardiac" phases, through the leafletchordal phase (redundancy), the myocardial phase (segmental left ventricular contraction abnormalities), to the anular phase (dilatation and faulty systolic contraction). Functional Anatomy is dealt with in terms of pathology, pathophysiology, hemodynamics, angiocardiography, echocardiography, and physical and pharmacological interventions. Clinical Manifestations are concerned with prevalence, natural history, symptoms, physical signs, electrocardiographic abnormalities and roentgen fingings. The four Major Complications- sudden death, infective endocarditis, spontaneous rupture of chordae tendineae, and progressive mitral regurgitation- are examined. Associated Cardiac Diseases, i.e., Marfan's syndrome, ostium secundum atrial septal defect and atherosclerotic coronary artery disease, are discussed, and a section on Treatment deals chiefly with prophylaxis for infective endocarditis and the management of arrhythmias and chest pain. A final section on Evolving Information considers etiologic concepts, the nature of left ventricular contration abnormalities, the cause of chest pain, the relationship to Marfan's syndrome and ostium secundum atrial septal defect, and the effect of aging and sex differences on leaflet chordal redundancy.