Synchronized direct current precordial electroshock has been established as a safe method for the reversion of ectopic cardiac arrhythmias to normal and is successful in 90% of all patients. Three or more months after reversion 40% of the patients with marked left atrial hypertrophy or atrial fibrillation of more than two years' duration remained in normal sinus rhythm. However, all patients with atrial flutter and 80% of those with atrial fibrillation of less than six months, in whom left atrial enlargement was minimal to moderate, remained normal. The safety of the procedure under the proper circumstances justifies an attempt at the correction of ectopic cardiac arrhythmias in all patients who will benefit from normal sinus rhythm.