Several studies have shown that dual chamber (DDD) pacemaker therapy relieves LV outflow obstruction and improves symptoms in most adult patients with obstructive hypertrophic cardiomyopathy (HCM). It is however, uncertain whether DDD pacing will be efficacious in children with obstructive HCM, because of evolving cardiac morphology and increased LV hypertrophy and outflow obstruction associated with rapid body growth. We propose to monitor clinical progress, and cardiac morphologic and hemodynamic changes over several years following implantation of a DDD pacemaker in children who present with obstructive HCM between the ages of 5 to 15 years. Functional status, myocardial ischemia, arrhythmias, and LV outflow obstruction will be evaluated by exercise tests, echocardiography, thallium scintigraphy, Holter monitoring, electrophysiologic and cardiac catheterization studies. The results of pacemaker therapy will be compared with the findings in a cohort of young patients with obstructive HCM who elect not to be treated with DDD pacemaker.