Blood viscosity, also known as blood stickiness, is an indicator of the fluidity of the body’s blood. There are two types of blood viscosity abnormalities: the first is an increase in blood viscosity. The second condition is a decrease in blood viscosity. In clinical practice, increased blood viscosity is somewhat more common. Increased blood viscosity can be caused by an increase in the number of blood cells or by changes in the solute composition of the plasma, such as in patients with erythrocytosis, thrombocytosis, and hyperleukocytic leukemia. As the number of blood cells increases abnormally, it causes a condition in which the viscosity of the blood increases significantly, predisposing it to hemosiderosis. Abnormally high blood viscosity can also occur in patients with increased levels of solute components in the plasma, such as hyperglycemia, hyperlipidemia, hyperfibrinogenemia, and multiple myeloma. The most common cause of decreased blood viscosity is a decrease in blood components, such as a significant decrease in blood viscosity in patients with severe anemia and a decrease in severe fibrinogen concentration.