Blood rheology is the science of the flow and deformation of blood. Clinically, blood rheology is important because circulatory resistance has two major components, vascular and rheological. In large vessels, blood rheology should be considered in terms of bulk flow, the viscosity of blood depending mainly on red cell concentration and plasma viscosity and, to a lesser extent, on red cell deformability and aggregation. In the microcirculation, where cells must deform to pass through narrow capillaries, cellular rheology (i.e. the deformability of individual cells) is a major determinant of resistance to flow. This ability to deform is also a determinant of the cell's survival time in the circulation. The deformability of the red cell is essentially linked to its structure (i.e. its cellular geometry, membrane properties and cytoplasmic viscosity); thus structural abnormalities, as found in some haematological disorders, can be expected to affect blood flow in the microcirculation and/or red cell lifespan. Blood rheology is a relatively new discipline as applied to the practice of haematology. In 1985 the International Committee for Standardization in Haematology (ICSH) established an Expert Panel on Blood Rheology which has subsequently issued guidelines on the measurement of blood viscosity and erythrocyte deformability and on tests such as erythrocyte sedimentation rate and plasma viscosity that are used to monitor the acute phase response in inflammatory disease. Rheological methods now have sufficiently good sensitivity and specificity for their application to a wide variety of clinical disorders. This review illustrates their potential application to haematological disorders that cause abnormal deformability of red cells.