The syndrome of primary mitral leaflet billow, with or without prolapse, is associated with myxomatous degeneration of the mitral valve apparatus, mainly the posterior leaflet, and the syndrome may be familial. It manifests clinically with an isolated nonejection systolic click (billow), a murmur of mitral regurgitation that is usually late systolic (prolapse), or a combination of murmur and click. Echocardiography identifies and assesses the extent of the billowing of mitral leaflet bodies but there are no specific echocardiographic criteria that can differentiate normal from pathological billowing. Similarly, a prolapsed leaflet is not detected echocardiographically when there is localized and mild failure of leaflet edge apposition but a more severely prolapsed, or flail, leaflet can be demonstrated and confirmed by that technique. Symptoms of the syndrome include anxiety, chest pain and palpitations. The resting electrocardiogram may show ST segment and T wave abnormalities. The majority of patients have a benign course and require reassurance only. Complications include systemic emboli, infective endocarditis, progression to severe mitral regurgitation, arrhythmias and, rarely, sudden death. Patients with prolapse of a leaflet edge are more likely to develop complications than those with only billowing of the leaflet bodies. Surgery, preferably valvuloplasty, is required for severe regurgitation and may also be indicated for potentially lethal tachyarrhythmias unresponsive to medical therapy. Mitral leaflet billow and prolapse may be secondary to, or associated with, many conditions. The prognosis is then principally that of the underlying disease of which ischemic heart disease and hypertrophic cardiomyopathy are the most important.