It is thought that mitral valve prolapse is the most commonly diagnosed valvular heart disease and affects 3 to 5% of the community. Mitral valve prolapse is commonly a benign disease and the prognosis is good. But it is understood that mitral valve prolapse causes sudden death rarely. As etiologies of their sudden deaths ventricular arrhythmia, mitral valve regurgitation et al. are known. Mitral valve prolapse is brought as the reason of sudden deaths in athletes not a little. Therefore, it is important to follow up them by exercise tolerance test, Holter ECG, ultrasoniccardiography, and chest auscultation et al. and the prescription of beta-blockade et al. and the insertion of implantable cardioverter defibrillator are considered. Moreover, it is important to decide right or wrong of exercise practice.