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Systolic anterior motion of the mitral valve and mitral-septal contact is the usual cause of dynamic left ventricular outflow obstruction in hypertrophic cardiomyopathy. That true obstruction actually occurs is now established based on cardiac catheterization and echocardiographic evidence. A mid-systolic drop in left ventricular systolic ejection velocity because of obstruction has been demonstrated recently. Echocardiographic data indicate that systolic anterior motion of the mitral valve is initiated by flow drag; the mitral valve is swept toward the septum by the pushing force of flow. After mitral-septal contact, obstruction begets further obstruction as the pressure gradient pushes the mitral valve into the septum. Most symptomatic patients with obstruction can be treated successfully with negatively inotropic drugs. These medications reduce systolic anterior motion and obstruction by decreasing early left ventricular ejection acceleration, decreasing the early systolic pushing force on the protruding mitral leaflet. Patients who do not improve on medication generally benefit from surgery. Newer interventions to relieve obstruction, such as dual-chamber pacing and percutaneous transluminal septal myocardial ablation are under active investigation.