Changes in blood flow are of particular importance in vascular diseases due to the perfusion-reducing effect of increased blood viscosity. Abnormal blood rheology found in occlusive arterial disease has been shown to be associated with the extent and the prognosis of the disease. Rheological disturbances in established acute deep venous thrombosis--probably reflecting a non-specific acute-phase reaction--might favor progression of the thrombus. Changes in plasma viscosity might play a particular role in microcirculatory disorders (diabetes mellitus, collagenoses).