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Leukocytosis is an elevation of the white blood cell count (the leukocyte count) above the normal range. The normal adult human leukocyte count in peripheral blood is 4.4-10.8 x 10⁹/L. A white blood count of 11.0 x 10⁹/L or more suggests leukocytosis.

Leukocytosis is very common in acutely ill patients. It occurs in response to a wide variety of conditions, including viral, bacterial, fungal, or parasitic infection, cancer, and exposure to certain medications or chemicals. Leukocytosis can also be the first indication of neoplastic growth of leukocytes.

A leukocyte count above 25 to 30 x 10⁹/L is termed a leukemoid reaction, which is the reaction of a healthy bone marrow to extreme stress, trauma, or infection. (It is different from leukemia and from leukoerythroblastosis, in which immature blood cells are present in peripheral blood.)

The mechanism that causes leukocytosis can be of several forms: an increased release of leukocytes from bone marrow storage pools, decreased margination of leukocytes onto vessel walls, decreased extravasation of leukocytes from the vessels into tissues, or an increase in number of precursor cells in the marrow.

Leukocytosis can be subcategorized by the type of white blood cell that is increased in number. Leukocytosis in which neutrophil count is elevated is neutrophilia; leukocytosis in which lymphocyte count is elevated is lymphocytosis; leukocytosis in which monocyte count is elevated is monocytosis; and leukocytosis in which eosinophil count is elevated is eosinophilia.

Despite its prevalence in acutely ill patients, leukocytosis in-and-of itself is not a disorder, nor is it a disease. Rather, it is resultant of the body's response to eliminate antigens (pathogens).

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