The electrocardiograms of 1,171 patients above the age of 65 in a predominantly geriatric institution were reviewed to determine the incidence of tachyarrhythmias. Data on the overall incidence and the individual types of arrhythmias are presented. Atrial fibrillation was the most common arrhythmia observed, followed by atrial flutter and supraventricular tachycardia. Atrial fibrillation often was associated with other evidence of myocardial damage. The significance of sinus bradycardia and grade I A-V block in the pathogenesis of atrial fibrillation and the significance of the tachyarrhythmias are discussed.