Blood viscosity, the principal physical property of each fluid, is the force acting against the flow which defines the flow conditions in whole circulatory system. Blood viscosity is influenced by external factors like temperature, flow velocity and oscillations but mainly depends on blood itself i. e. the value of hematocrite, red blood cell deformability and plasma proteins composition. Hematology was the first branch of medicine interested in blood viscosity thus blood viscosity disturbances are most widely described in hematological disorders. Composite haemorheologic disturbances are observed in numerous cardiovascular diseases too. Importance of the blood viscosity is emphasized in the pathogenesis of atherosclerosis, and its cardio-vascular complications including acute myocardial infarction and ischaemic stroke as well as in diabetes, metabolic syndrome and hyperlipidemia. Blood viscosity disturbances are commonly considered as clinical steady components of the arterial hypertension especially in the more advanced stages (ESH/ESC 2 and 3 stage). Factors responsible for elevated blood viscosity in patients with arterial hypertension include elevated hematocrite values and hyperfibrynogenemia. According to Poiseuille’s law, blood viscosity increases systemic blood pressure via increased vascular resistance. On the other hand blood viscosity is necessary for shear stress forces development between blood and vascular wall. Shear stress acts on vascular endothelium where increases the synthesis of vasodilators (mechanotransduction). The role of blood viscosity in blood pressure regulation is composite. Pathological increase in blood viscosity is responsible for hypertensive complications and remains in strong correlation with total mortality among hypertensive patients. The influence on blood viscosity can be important in antihypertensive therapy. The influence of new antihypertensive drugs on blood viscosity is considered to be positive. However, results from the research carried out up to now need to be confirmed in prospective randomized clinical trials.