Abstract The records of five patients with idiopathic atrial fibrillation were reviewed. These cases were selected on the basis of the absence of any heart disease or other condition that might have accounted for the arrhythmia. No significant changes were present in the coronary arteries in any of the cases. No symptoms marked the onset of the fibrillation. Anatomically, the weight of the heart was greater than normal in all five cases. Left atrial enlargement and endocardial thickening were also present in all cases. Intracardiac thrombosis was noted in only one case and was associated with multiple peripheral emboli. In two cases the cause of death was thought to be related to the chronic atrial fibrillation. It is suggested that idiopathic atrial fibrillation may go unrecognized in some persons until later life, when some form of heart disease develops that is then taken to be the cause of the arrhythmia.