The efficacy of oral anticoagulation therapy in reducing the risk of embolic stroke in patients with atrial fibrillation and rheumatic heart disease was well known. The value of anticoagulant therapy in patients with atrial fibrillation without rheumatic heart disease had not been established. Several studies strongly suggested that although the risk of stroke in patients with atrial fibrillation was greatest in the presence of valvular disease, the risk of stroke in the absence of valvular disease was also much greater in patients with atrial fibrillation than those without this arrhythmia. What was lacking was a detailed controlled study assessing the degree of reduction in stroke risk by anticoagulation of fibrillating patients without valvular disease.