The physiology of the blood and the vascular system suggest that the rheological properties of blood gain in importance as arteriosclerotic vascular changes progress. Thus, for example, in arterial segments distal to a stenosis, a decrease in perfusion pressure (in the nutritive vessels) occurs--a situation that is probably of causative relevance in a considerable percentage of patients with apoplexy. An increasing number of studies seem to indicate that blood and plasma viscosity, red cell aggregation and fibrinogen levels in these patients reveal significant pathological changes, and may well be of prognostic significance. It may be speculated that routine monitoring of these and possible other rheologically important parameters in patients with a relevant risk together with suitable treatment may contribute to more effective primary and secondary prevention of cerebrovascular diseases, in particular ischemic insults.