Main causes of cerebral infarction and focal transient cerebral attacks (TIAs) were evaluated. We distinguish between primary thrombosis, secondary thrombosis, and atherosclerotic embolism (platelet emboli-mixed emboli-atheromatous emboli). As to the haemodynamic significance of atherosclerotic lesions, the stenoses must be very tight, i.e., suppress 80--90% of the arterial lumen, for blood flow to be reduced. In clinical trials such a situation is not common and we rarely have found out that a stenosis has caused an infarct or focal TIA by reducing blood flow.