OF ALL THE ARRHYTHMIAS, atrial fibrillation is the one most frequently associated with organic heart disease and heart failure. Moreover, it responds most satisfactorily to therapy, often with dramatic results. Two types of atrial fibrillation are encountered: paroxysmal and established. The condition occurs most frequently after the age of 40. Rarely seen in infants, it is not uncommon in older children, predominantly in females; later in life the incidence is higher in males. Atrial fibrillation is almost always observed in the presence of myocardial disease, especially of an advanced form. It occurs in about 60% of patients with heart failure. The etiological factors are varied. In a general hospital sample, the hypertensive, arteriosclerotic-heart-disease group ranks first in frequency, with the rheumatic-heart-disease group second: Other less common factors include thyrotoxicosis, toxic factors, and association with congenital cardiac anomalies. Atrial fibrillation, as well as other arrhythmias, is seen not infrequently during surgery