Chronic atrial fibrillation (AF) as a precursor of stroke was assessed over 24 years of follow-up of the general population sample at Framingham, Massachusetts. Persons with chronic established AF, with or without rheumatic heart disease (RHD), are at greatly increased risk of stroke, and the stroke is probably due to embolism. Chronic AF in the absence of RHD is associated with more than a fivefold increase in stroke indicence, while AF with RHD has a 17-fold increase. Stroke occurrence increased as duration of AF increased, with no evidence of a particularly vulnerable period. Chronic idiopathic AF is an important precursor of cerebral embolism. Controlled trials of anticoagulants or antiarrhythmic agents in persons with chronic AF may demonstrate if strokes can be prevented in this highly susceptible group.