In order to determine possible correlations to the extent of extracranial carotid artery disease (ECAD), the hemorheologic parameters (hematocrit, fibrinogen, whole blood and plasma viscosity, induced platelet aggregation) of 120 patients with acute cerebrovascular disorders (CVD) were studied and their carotid arteries were investigated by means of Duplex scanning. Plasma viscosity was only significantly elevated in CVD patients with severe ECAD (stenosis >80°/o lumen reduction and/or occlusion) in comparison to CVD patients with normal to mimimally diseased carotids and 67 normal controls. These results suggest that high plasma viscosity is an indicator for the existence of severe ECAD in CVD patients during the acute stage