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 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.

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 likely cause of leukocytosis can be narrowed down by measuring what types of white blood cell have an increased level. 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.