Cardiogenic and aortal embolias are a clinically important source of brain stroke. They occur most frequently in patients with atrial fibrillation, and their prevalence in the group not receiving antithrombotic treatment amounts to about 5 per cent yearly. Cerebral embolism is also a complication of a recent myocardial infarction. In the latter case the embolus consists of blood clots forming in the left ventricle of the heart. Two other groups at risk for cerebral embolism are, firstly, patients with bacterial endocarditis, especially prior to antibiotic treatment, and secondly, those with artificial heart valves who for this reason should have a maintenance antithrombotic treatment for life. Transoesophageal echocardiography has indicated other sources of cerebral embolia, such as paradoxical embolus or aorto-arterial embolus, but their clinical importance requires further study.