This study will review medical information collected on children and adolescents with hypertrophic cardiomyopathy (HCM) to try to identify risk factors for arrhythmias (abnormal heart rhythms) in these patients and better guide the choice of treatment options for them. Arrhythmias arising from the ventricle (lower heart chamber) can cause dizziness, fainting or cardiac arrest. Predictors of arrhythmias in adult HCM patients may not apply to children and teenagers with HCM.