The electrocardiogram is a valuable aid in diagnosing and treating heart disease in the elderly and in determining the prognois. In general, mortality is lower in old persons with ECG abnormalities than in younger ones, and some changes are not considered abnormal. P wave notching, slurring, and loss of amplitude are too common to be of diagnostic significance, and T wave inversions and S-T segment depressions have too many causes to be specific. A persistently prolonged P-R interval often is associated with recurrent atrial arrhythmias, junctional rhythms, and conduction distrubances. A short interval, on the other hand, may be noted for a long time without paroxysmal atrial tachycardias. Degenerative and ischemic changes in the conduction system result in a variety of arrhythmias and conduction disturbances. The classic sick sinus syndrome is not seen as often as the incomplete forms, such as sinus bradycardia with atrial premature contractions. With the exception of left inferior hemiblock, bundle-branch blocks are common. Similarly, ventricular and atrial premature contractions increase with age. ECGs taken routinely over a period of years help differntiate ventricular from supraventricular tachycardias with aberrant conduction.