About three times as many patients with rheumatic heart disease and auricular fibrillation have intracardiac thrombi as those with normal rhythm (table 1, 45.7 and 15.7 per cent). The arrhythmia, and not factors associated with age, apparently is the main factor causing the thrombus.1According to the available data, auricular fibrillation is responsible for extra clot formation in 25 to 30 patients of 100 with rheumatic heart disease. The prevention of this formation of antemortem clots could theoretically be accomplished by continuous anticoagulant treatment following the establishment of the arrhythmia. The recognized hazards of dicumarol® have usually restricted its use in auricular fibrillation to those patients gravely sick with repeated embolic episodes. For such patients the drug has proved gratifying in the prevention of further thromboembolism.2Its use apparently prevents the propagation of already formed thrombi. The prevention of formation of primary intracardiac thrombi, too, theoretically is possible.