A study of the blood platelets yields useful knowledge for guiding treatment and appraising the state of health of a patient with leukemia. In chronic myelogenous leukemia the numbers of these elements may be normal, enormously increased, or greatly diminished, and in chronic lymphatic and acute leukemia it is the rule to find them below normal. The fluctuations that occur in the numbers of blood platelets are discussed. Petechiæ and other hemorrhages associated with decrease of blood platelets are common, and hemorrhages, but not petechiæ, may occur in chronic myelogenous leukemia when the platelets are much increased.