Myotonic dystrophy type 1 (MD1) is an autosomal dominant disorder characterized by myotonia, progressive muscular weakness, cataract, and cardiac involvement. Cardiac involvement is common and includes conduction system abnormalities, supraventricular and ventricular arrhythmias, and less frequently, myocardial dysfunction and ischemic heart disease. A 54-year-old woman with a previous diagnosis of MD1 was admitted with palpitation, blood pressure of 157/118 mmHg, and a heart rate of 220 beats/min. Electrocardiography (ECG) showed ventricular tachycardia. Within minutes, hemodynamic collapse developed and electrical cardioversion was performed. Immediately following cardioversion, ECG showed atrial fibrillation, a slightly prolonged QT interval, and intraventricular conduction delay. After intravenous infusion of amiodarone, the rhythm converted to sinus. Transthoracic echocardiography showed significantly depressed left ventricular function, an ejection fraction of 25%, and normal coronary arteries. During electrophysiological study, atrium-His interval and His-ventricle interval were 120 msec was 54 msec, respectively, and monomorphic ventricular flutter was induced. An implantable cardioverter-defibrillator was placed. She was discharged in sinus rhythm.