Among 909 patients with acute myocardial inarction treated in an intensive care unit between 1970 and 1974, atrial flutter and (or) fibrillation occurred in 124 (13.6%). The incidence of these arrhythmias rose with increasing age and predominantly in paroxysmal form (78%). The clinic mortality of patients with arrhythmias was 42%, while in the remaining 785 it was only 26% (P less than 0.001). Patients with atrial fibrillation and (or) flutter had a higher mean age, more frequently cardiac failure (P less than 0.001) - especially in the prognostically unfavourable severe forms with pulmonary oedema (P less than 0.05) and combined right and left heart failure (P less than 0.001) - and other disorders of impulse conduction or formation and chronic arterial hypertension (P less than 0.01).