The incidence of stroke in patients with atrial fibrillation or flutter (AF) is well known and depends on the presence of risk factors.   The incidence of AF in patients who have a stroke and its relationship to risk factors is not clear, however, because many stroke patients may have occult intermittent AF that is not present at the time of stroke and is not diagnosed.   To better assess the incidence of AF, we studied the clinical records and all the 12 lead ECGs in a 14 year medical center data base of 985 patients admitted with ischemic stroke over a 3 year period and correlated the incidence of AF with the presence of the stroke risk factors.  Of the stroke patients with congestive heart failure 61.9% had AF (95%CL 54.4, 68.9), with age >/= 75 years 45.2% had AF (CL 41.0,49.4), with coronary artery disease 42.9 had AF (CL 36.8, 49.2), with diabetes 39.2% had AF (CL 32.8,46.1) and with hypertension 33.7% had AF (CL 30.5, 37.1), all significantly higher than without these risk factors.   Patients with more than one risk factor or with echo abnormalities, especially left atrial enlargement, had an even higher incidence of AF.   These findings suggest that AF may be a very common mechanism whereby the stroke risk factors cause stroke.   Stroke patients in normal sinus rhythm with these risk factors should be monitored for AF so they can receive anticoagulation to prevent a subsequent stroke if AF is diagnosed.