Impaired blood flow due to abnormal rheologic characteristics results in a multiplicity of clinical manifestations, collectively termed the hyperviscosity syndrome. A basic knowledge of the principles of rheology is important in the understanding of its pathophysiology, especially the relationship between viscosity and flow conditions. The flow characteristics in different types of blood vessels are also determinants in the location of the clinical manifestation. The syndrome can occur in a wide variety of diseases and is best grouped according to the causative element or elements in blood. Abnormalities in the cellular components of blood can occur in the quantity and the quality of erythrocytes, leukocytes, and platelets. Abnormal plasma components can also be in both the quantity and quality of the plasma proteins. Clinical manifestations are the result of vascular occlusion, especially in the microcirculation. The altered rheologic characteristics of either the cellular or the protein component may be temperature dependent, being abnormal only at temperatures below 37°C, so that only the cooler parts of the body are affected. The management of these conditions should be primarily directed at the removal of the abnormal component. At the same time, it should be accompanied by measures that can control the production of the causative element.