DDD pacing improves symptoms and relieves LV outflow tract (LVOT) obstruction in most patients with hypertrophic cardiomyopathy (HCM). Notably, when pacing is temporarily discontinued, the beneficial effects of pacing are evident in sinus rhythm. The long term results of this novel therapy are, however, uncertain. We propose (1) to record the hemodynamic changes following >4 years of pacing; and (2) to determine whether DDD pacing continues to be necessary in patients who have had a substantial relief of their LVOT obstruction. Patients who have had >50% reduction in LVOT pressure gradients will be randomized to two pacing modalities: DDD at 70 beats per minute and AAI pacing at 70 beats per minute (DDD switched off), and reevaluated after a six-month period.