On retrospective review of records of 150 patients with atrial fibrillation (AF), 31% experienced a stroke or peripheral embolism. The AF was not associated with cardiac valvular disease in 80% of the 150 patients. Most of the cerebral infarcts were large, disabling, and unheralded by transient ischemic attack. The thromboembolism typically occurred in patients whose AF was undetected prior to the infarction. Half of the patients with an ischemic event suffered multiple events, with one fourth of the recurrences arising within two weeks.