The rheological behavior of blood and its components under physiologic and pathophysiologic conditions is reviewed, with a focus on the type and extent of pathohemorheological changes in surgical patients during hospitalization and thereafter, as well as their clinical consequences with regard to thromboembolic complications. During the operation and the postoperative period various hemorheological and hemostasiological alterations acquire clinical significance: 1. hyperreagibility of platelets with increased aggregation and adhesion tendency 2. changes in fibrinogen, albumin, and globulin concentrations, which affect viscosity and red cell aggregation 3. impairment of red cell deformability 4. increase in clotting factors 5. disturbance of fibrinolysis characterized by diminution of plasmatic plasmin and increase in antiplasmin activity In addition, anesthetic techniques have also been shown to affect hemorheological and hemostasiological parameters. The complex pattern of pathohemorheological and hemostasiological changes shows that thromboembolism in the course of surgical interventions is provoked by multifactorial disorders. Thrombosis prevention should, therefore, counteract both hemorheological and hemostasiological disturbances. Since pathohemorheological and pathohemostasiological changes are already detectable before, and increase during operation, preventive measures should start before the surgical intervention to obtain maximum benefit. Therapeutic possibilities for the avoidance of these multifactorial disturbances are discussed with particular reference to pentoxifylline, which satisfies the complex requirements of a hemorheologically and hemostasiologically active therapeutic agent.