Patients with obstructive HCM and drug-refractory symptoms are referred for left ventricular myotomy and myectomy (LVMM) or mitral valve replacement. As alternative therapies to cardiac surgery, we propose to compare the abilities of dual chamber (DDD) pacing and percutaneous transluminal septal ablation (PTSA) to reduce left ventricular (LV) outflow pressure gradients and to improve exercise performance in patients with obstructive HCM and severe symptoms who have failed to benefit from pharmacotherapy.