Studies suggest that DDD pacemaker therapy is effective in improving symptoms and reducing intra-ventricular pressures and pressure gradients in children with obstructive HCM during a period of rapid body growth when the severity of the disease is expected to worsen. We propose a randomized, cross-over study of DDD pacing versus placebo (AAI pacing mode) in children who have limited exercise performance and/or symptoms despite medical therapy (beta-blocker or verapamil). Study subjects will receive a pacemaker and will be randomized to one of two pacing modes (DDD, AAI). After a 1-year follow-up evaluation the children will cross-over to the alternative pacing mode, and will be re-evaluated after a further year. All children will continue to receive optimum medical therapy (beta-blocker or verapamil) based on improvement in symptoms and exercise performance. The subjects will undergo outpatient evaluations (exercise tests and echocardiography) 3, 6, 15, and 18 months after pacemaker implantation, and inpatient evaluations (exercise tests, echocardiography, and cardiac catheterization) 1 year and 2 years after entry into the study. Primary end-points will be exercise duration and severity of LV outflow obstruction. The patient, parents, referring physician, and individual supervising the exercise tests will be blinded to the pacing mode.