Hemorheology deals with interactions between blood flow and hematology, some of which are discussed in this review. In arteries erythrocytes flow in the centre, while platelet-rich plasma flows along the vessel wall. Blood viscosity is determined by hematocrit, erythrocyte aggregation and deformation, and plasma viscosity. A hyperviscosity syndrome is most often seen when plasma viscosity is severely increased by paraproteinemia. Among the blood cells erythrocytes are most relevant. Their membrane skeleton provides them with an astonishing deformability and stability. Defects in the membrane skeleton lead to hemolytic anemia: in the case of a deficiency in the attachment of the skeleton to the lipid bilayer, spherocytosis develops, while in the case of defects within the skeleton with lateral instability, elliptocytosis or poikilocytosis develops. Sickle cell disease is a classic example of a hemorheological disorder with microcirculatory plugging by irreversibly sickled erythrocytes and abnormal adhesion of deformable sickle cells to venular endothelium. Leukocytes are much less deformable than erythrocytes, which is even aggravated when neutrophils are activated. When present in large numbers, leukocytes may cause the leukostasis syndrome, a plugging of the microcirculation. Neutrophils are capable of leaving the flowing blood by selectin-mediated rolling on venular endothelium followed by firm adhesion through integrins, which is a prerequisite for diapedesis. High shear forces induce platelet aggregation. Thus the composition of a thrombus depends on local flow conditions.(ABSTRACT TRUNCATED AT 250 WORDS)