Sudden cardiac death is the most devastating complication of hypertrophic cardiomyopathy. Since HCM may present at young age, and since the risk period for sudden arrhythmic death may be long, decision-making in HCM patients may be difficult, and have lifelong imp- lications. Community based studies show a sudden death mortality of approximately 1%/year. Certain patients can be identified by their clinical characteristics, and through testing, to have higher annual risk, as high as 4-5%/year. Risk factors for sudden cardiac death include: family history of HCM and sudden death, recurrent syncope, ventricular tachycardia, as detected by Holter monitoring or exer- cise testing, subnormal (<20 mmHg) increase in systolic blood pressure on maximal exercise testing and lastly marked (especially >30 mm) left ventricular hypertrophy. The implantable defibrillator has been shown to avert sudden death in selected HCM patients deemed to be at high risk.