Atrial arrhythmias occurred in 34 out of 97 patients with endomyocardial fibrosis. In 21 instances, atrial fibrillation or flutter was persistent. In 13 cases, atrial arrhythmia was transient and intermittent. It was necessary, in some patients with rapid ventricular re sponses, to employ direct-current cardioversion. Because of the severity of endomyocardial fibrosis at the time of clinical observation, further deterioration at the onset of an atrial arrhythmia was not impressive.