A randomized multicenter trial. The trial enrolled only patients with atrial fibrillation who were at high risk for stroke, that is, over 65 years of age or less than 65 and with one or more other risk factors for stroke such as systemic hypertension, diabetes mellitus, congestive heart failure, transient ischemic attack, prior cerebral vascular accident. High risk patients were treated with the anticoagulant warfarin. Cardioversion (electrical or pharmacologic) might have been attempted before randomization, but if it was unsuccessful, the patient was excluded from further consideration for randomization. Normal sinus rhythm must have persisted for one hour or greater after cardioversion to qualify as successful cardioversion. Patients were randomly assigned to treatment groups which included maintenance of sinus rhythm or heart rate control. Both treatment groups had two steps.