A leukemoid reaction is a leukocytosis exceeding 50 000/µL due to a cause other than leukemia. This is typically characterized by elevated neutrophils, indicative of an acute inflammatory process, and the absence of dysplastic cells suggestive of a hematologic malignancy. Coinciding with a marked increase in leukocytes is the presence of neutrophil precursor cell types: myelocytes, metamyelocytes, and promyelocytes.

Leukemoid reactions occur in a variety of different conditions, all indicative of acute stress. These include infectious etiologies, adverse reactions to medications, asplenia, diabetic ketoacidosis, and septic shock with end-organ damage. Infants with trisomy 21 can often be born with a leukemoid reaction.

Leukemoid reactions will resolve with treatment of the underlying condition.