Left ventricle myotomy and myectomy was carried out in nine patients with obstructive hypertrophic cardiomyopathy solely because of a previously documented episode of cardiac arrest. Before cardiac arrest, each patient had either no or only minimal functional limitation and therefore would not have met the usual criteria for operation, namely, severe symptoms unresponsive to medical therapy. Of the nine patients, one died in the perioperative period, and one died suddenly and unexpectedly 9 months postoperatively. The remaining seven patients have survived 9 months to 5.5 years after operation; six of the seven are asymptomatic and one has only mild symptoms. Operation resulted in a marked decrease or abolition of the left ventricular outflow gradient under basal conditions in seven of the eight patients studied postoperatively. Significant residual outflow obstruction was demonstrated after operation in the patient who later died. Sudden death in patients with obstructive hypertrophic cardiomyopathy appears usually to result from ventricular arrhythmia, and prevention of recurrent fatal arrhythmia is the goal of treatment in patients who have had cardiac arrest and have been resuscitated. Such patients should be treated indefinitely with antiarrhythmic drugs. In addition, when severe outflow obstruction is present, we postulate that effective operative relief of obstruction and consequent reduction of left ventricular systolic pressure will provide additional protection.