In 39 patients with hemodynamically significant stenoses of the internal carotid artery thrombendarterectomy was performed. Hemorheological properties and plasma protein levels were studied pre- and postoperatively over a period of 14 days. In comparison to age-matched healthy controls preoperative plasma viscosity, red cell aggregation, fibrinogen, haptoglobin and alpha-1-antitrypsin were significantly elevated. After surgical trauma there was an increase in fibrinogen, alpha-1-antitrypsin, haptoglobin and ceruloplasmin, whereas alpha-2-macroglobulin, immunoglobulin A and immunoglobulin G decreased significantly. Plasma viscosity, red cell aggregation, red cell filterability, fibronectin and immunoglobulin M remained unchanged. We conclude that our patients showed a non specific chronic "hematological stress syndrome" with raised acute phase reactants and corresponding hemorheological changes preoperatively; despite postoperative acute phase reaction no further deterioration of the rheological parameters could be observed; this might be caused by a decrease of plasma proteins with high frictional ratios compensating the increase of acute phase proteins. Besides, tissue damage and consecutive acute phase reaction in carotid arterial surgery seem to be of relatively minor degree.