Increase in blood viscosity, defined as resistance to flow, is one factor in hypertension and atherosclerosis that contributes to the morbidity and mortality associated with tissue ischemia. In this research we evaluated the effect of hematocrit on increasing viscosity, and possible related changes in blood pressure, flow rate, and the equivalent physiologic compensation ratios. Blood samples were taken from 32 healthy individuals and centrifuged for 5 min at 3000 rpm to obtain 2.5 mL of erythrocyte mass from each. Then, at each step 0.5 mL of plasma was consecutively added in a total of 17 steps. The resultant hematocrit and viscosity changes were measured. Viscosity measurement was performed by capillary viscometer. The results were evaluated by the Student t test. It was observed that in the range of 60.16% and 25.32%, a 10.99% increase of hematocrit produced an increase of 1 unit relative viscosity, which means approximately a 20% increase in blood viscosity for a healthy individual. According to Poiseuille's equation, with a constant vessel length, if viscosity is increased by 20%, the decrease in blood flow rate will be 16.67% (100/120 = 83.33%; 100 - 83.33 = 16.67%). For the physiologic compensation of 20% increased viscosity, blood pressure increase will be 20% or vasodilation will be 4.66% in radius. Atherosclerotic and some healthy vessels with little vasodilatory capacities might benefit from treatment modalities to decrease the viscosity by hemodilution.