Sudden death is an important facet of the natural history of hypertrophic cardiomyopathy. Issues related to the nature and etiology of sudden death and the prospective identification of those patients at increased risk have been the subject of intense study, and consequently, our concepts and knowledge have continued to evolve. Occurrence of sudden death has been reported to be about 2-3% per year in a hospital-based referral population and, although described in most age groups, is most common in older children and young adults. The typical profile is that of a young asymptomatic patient with substantially increased left ventricular wall thickness who dies while performing sedentary or modest physical activities; however, a substantial minority die suddenly during or just after severe exertion, including those participating in competitive athletics. Other risk factors that have been identified in patients with hypertrophic cardiomyopathy include nonsustained ventricular tachycardia on ambulatory electrocardiogram, a strong family history of sudden death, and prior occurrence of syncope (or cardiac arrest). Electrophysiological studies have shown that most patients judged at increased risk for sudden death have sinoatrial or His-Purkinje conduction disease or inducible supraventricular or ventricular tachycardia; inducibility of a sustained ventricular arrhythmia is associated with prior occurrence of syncope or cardiac arrest. Hemodynamic and electrophysiological studies in patients with hypertrophic cardiomyopathy have demonstrated several potential mechanisms for cardiac arrest or sudden death, including atrial arrhythmias associated with hypotension, bradyarrhythmias, and ventricular tachyarrhythmias, all of which can be exacerbated in the presence of left ventricular outflow tract obstruction or myocardial ischemia.