A small proportion of patients with acute or chronic leukemia has an extraordinarily high blood leukocyte count. These high counts can result in a very high fractional volume of leukocytes (leukocrit), which is also a function of the mean leukocyte volume in different types of leukemia. Despite a high fractional volume of leukocytes, bulk viscosity of blood is usually not increased because a decrement in the fractional volume of erythrocytes accompanies the increase in leukocytes. Nevertheless, the excessive numbers of leukocytes present two major problems: first, they can seriously affect flow in the circulation of the lung, brain, and less often, other organs by obstructing microchannels or by forming aggregates and white thrombi in small veins. Moreover, leukemic blasts may compete for oxygen in the microcirculation and they may be invasive, damaging vessel walls. Second, their rapid destruction in response to cytotoxic drugs causes metabolic disturbances, especially uric acid accumulation, which can lead to obstructive uropathy.