The long-term pattern of prednisone-induced leukocytosis was examined in 80 patients. Our results disclosed an extremely variable leukocytic response, in which the white blood cell count surpassed 20,000/mm3 as early as the first day of treatment, an increase that persisted for the duration of therapy. Although the degree of leukocytosis was related to the dosage administered, it did appear sooner with higher doses. Leukocytosis reached maximal values within two weeks in most cases, after which the white blood cell count decreased, albeit not to pretreatment levels. The leukocytosis was attributed predominantly to a rise in the polymorphonuclear white blood cells, a phenomenon that coincided with monocytosis, eosinopenia and a variable degree of lymphopenia. It can be concluded that even small doses of prednisone, administered over a prolonged period of time, can induce extreme and persistent leukocytosis. This observation is of consequence especially when infection is suspected, particularly in an immunocompromised host. However, a shift to the left in the peripheral white blood cells, i.e., more than 6 percent band forms, and the appearance of toxic granulation may assist in the differential diagnosis beween infection, in which the latter are observed, and corticosteroid-induced leukocytosis, in which they are rare.