Causative factors, clinical consequences and treatment of atrial tachyarrhythmias were reviewed in 917 monitored patients with definite acute myocardial infarction. Significant atrial tachyarrhythmias were found in 104 (11 per cent) of them and included atrial fibrillation in 67, atrial flutter in 29 and paroxysmal atrial tachycardia in 33. These episodes were single in 79 patients and multiple in 25, and began within the first four days of acute myocardial infarction in 90 per cent of the patients. Fifty per cent of these atrial tachyarrhythmias were heralded by premature atrial contractions. The incidence of atrial tachyarrhythmia was not related to the location of the acute myocardial infarction or to the presence or degree of power failure; however, atrial tachyarrhythmias were significantly more frequent in patients with pericarditis. Atrial tachyarrhythmias were well tolerated in almost one fifth of the patients, caused marginal compromise in almost two thirds and led to severe clinical deterioration in one fifth. Paroxysmal atrial tachycardia rarely required specific treatment, atrial fibrillation was best managed with intravenous administration of digoxin except when associated with severe clinical compromise, and atrial flutter generally required cardioversion or rapid intravenous therapy and usually caused severe clinical deterioration. Over-all, atrial tachyarrhythmia was not associated with a significantly increased mortality, and in those who died, death was not related specifically to the atrial tachyarrhythmia but rather to the severity of the underlying acute myocardial infarction. However, persisting atrial tachyarrhythmias, particularly atrial flutter which tends to be refractory to both heart rate control and cardioversion, may contribute indirectly to morbidity and mortality.