The mechanism(s) of myocardial ischemia in hypertrophic cardiomyopathy remain unclear. In this report, the authors present a 75-year-old Japanese woman with nonobstructive hypertrophic cardiomyopathy in whom paroxysmal atrial fibrillation caused severe myocardial ischemia and induced sustained ventricular tachycardia. Her coronary angiogram showed normal findings, and no ischemic changes were provoked by either physical exercise testing or dobutamine stress echocardiography under sinus rhythm. In view of these findings, the rapid ventricular response in the absence of atrial contraction may aggravate or induce myocardial ischemia and predispose patients with hypertrophic cardiomyopathy to develop lethal ventricular arrhythmia.