The ECG is useful in diagnosing acute myocardial infarction and unrecognized Q-wave myocardial infarction in the elderly. Unrecognized myocardial infarction and myocardial infarction associated with clinical symptoms have a similar incidence of new coronary events. Ischemic ST-segment depression on the resting ECG is associated with an increased incidence of new coronary events. The ECG is useful in the diagnosis of LV hypertrophy but is less sensitive and less specific than echocardiography in diagnosing LV hypertrophy. ECG LV hypertrophy is associated with an increased incidence of cardiovascular events in the elderly. However, echocardiographic LV hypertrophy is more sensitive in predicting new coronary events, atherothrombotic brain infarction, and congestive heart failure than is ECG LV hypertrophy. The ECG is also useful in diagnosing conduction defects and arrhythmias in the elderly. In the elderly, left bundle branch block, intraventricular conduction defect, Type II second-degree atrioventricular block, and pacer rhythm are associated with an increased incidence of new cardiac events, whereas right bundle branch block, left anterior fascicular block, and first-degree atrioventricular block are not. In the elderly, atrial fibrillation is associated with an increased incidence of thromboembolic stroke and new cardiac events. Premature atrial complexes and paroxysmal supraventricular tachycardia are not associated with an increased cardiac risk. Complex ventricular arrhythmias on the resting ECG are associated with an increased incidence of cardiac events in elderly patients with heart disease but not in elderly patients without heart disease.(ABSTRACT TRUNCATED AT 250 WORDS)