Hypertrophic cardiomyopathy (HCM) is the most common cause of sudden cardiac 
death in young people, including trained athletes. The implantable 
cardioverter-defibrillator (ICD), although initially designed as a treatment for 
older patients with coronary artery disease, has more recently proved to be a 
safe and effective therapeutic intervention in young patients with HCM, both for 
primary or secondary prevention of sudden death. The largest such report of >500 
patients showed that the ICD intervened appropriately to abort ventricular 
tachycardia/fibrillation (VT/VF) in 20% of patients over an average follow-up 
period of only 3.7 years, at a rate of about 4% per year in those patients 
implanted prophylactically, and often with considerable delays of up to 10 
years. Extensive experience with high-risk HCM patients showed that appropriate 
device discharges for VT/VF occur with similar frequency in patients with 1, 2, 
or > or = 3 noninvasive risk markers. Despite the extreme morphology 
characteristic of HCM, often with massive degrees of left ventricular (LV) 
hypertrophy and/or LV outflow tract obstruction, ICDs performed in a highly 
effective fashion, with failure to convert life-threatening arrhythmias 
extraordinarily rare. In conclusion, in a large high-risk HCM cohort, ICD 
interventions for life-threatening ventricular tachyarrhythmias were frequent 
and highly effective in restoring normal rhythm. An important proportion of ICD 
discharges occurred in primary prevention patients with only one risk factor. 
Therefore, a single marker of high risk may represent sufficient evidence to 
justify the recommendation for a prophylactic ICD in selected patients with HCM.