Cardiac illness in myotonic muscular dystrophy (MyD) is infrequent, but subclinical cardiac involvement in MyD is very common (found in 42 of 46 subjects) and may be responsible for sudden death. In this series, we found ECG abnormalities in 72%, left ventricular dysfunction in 70%, mitral valve prolapse in 37%, and sudden death in 4%. Four deaths during the study period were due to acute left ventricular failure, one to sepsis and respiratory insufficiency, and one was unexplained. We did not find ominous bradyarrhythmias or atrioventricular block, evidence of congestive heart failure, noninvasive evidence of coronary artery disease, or any correlation of type or amount of cardiac involvement with any clinical parameter such as age, sex, or severity of systemic dystrophy. We feel tachyarrhythmias may play as important a role in sudden death of myotonic muscular dystrophy subjects as bradyarrhythmias, and coronary artery disease in addition to cardiac dystrophy may produce arrhythmias and myocardial dysfunction in myotonic muscular dystrophy. In addition, some subjects have an unusual form of resting left ventricular dysfunction which improves with exercise. The most important problem in the clinical management of myotonic muscular dystrophy subjects is sudden death, and the solution does not appear to be empiric ventricular pacing. Our recommendations for prophylaxis of sudden death in myotonic muscular dystrophy are noninvasive investigation of coronary artery disease in subjects with significant risk factors, with angiography and surgery if indicated: detailed evaluation of syncopal and presyncopal events, including electrophysiologic testing, with pacemaker or antiarrhythmic drug therapy if indicated; and consideration of ventricular pacing of asymptomatic subjects if severe bradycardia or marked intraventricular conduction delay develops during follow-up, serial 12-lead ECGs. The documentation of tachyarrhythmias during sudden death and syncopal episodes in myotonic muscular dystrophy subjects makes ventricular pacing alone an uncertain modality for prevention of sudden death in subjects with only mildly lengthened PR or QRS intervals, and suggests a combination of pacemaker and antiarrhythmic drug therapy for the myotonic muscular dystrophy subject with syncope of no apparent cause.