Analysis of published and author's data on the physiological role of deformability of erythrocytes, general mechanisms of modifications and disturbances, assessment methods, and the role of this blood feature for assessing the body status. This parameter is one of the most labile features of blood that shows highly sensitive reaction to practically any changes in metabolic process in erythrocytes and in a whole body. Degradation of deformability of erythrocytes under various types of oxygen deficit deteriorates functioning of the system of oxygen transportation at various levels: heart, vascular flow, oxygen-transporting blood function. Under hypoxia the parameters of oxygen-transporting blood function, of peroxide oxidation of lipids and of the antioxidation system correlate well with degradation of deformability of erythrocytes. Therefore, this parameter can be used an integrated criterion of disturbances in oxygen supply, and of prooxidation-antioxidation body status. Deformability of erythrocytes is a factor generating adequate the oxygen supply to tissue, and its degradation aggravates substitution of the oxidase pattern of oxygen utilization with the oxygenase pattern. This parameter is extremely important for the body functional status.