A leukemoid reaction is an increase in the white blood cell count, which can mimic leukemia. The reaction is usually due to an infection or inflammation and is not a sign of leukemia. It can occur due to cigarette smoking, certain medicines, heart attack and certain cancers.
Blood counts often return to normal when the underlying condition is treated.
References
Berliner N. Leukocytosis and leukopenia. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 153.
Nasr MR, Hutchison RE. Leukocytic disorders. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 34.
Rice L, Zieske AW, Jung M. Neutrophilic leukocytosis, neutropenia, monocytosis, and monocytopenia. In: Hoffman R, Benz EJ, Silberstein LE, et al, eds. Hematology: Basic Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 49.
Review Date 5/1/2026
Updated by: Warren Brenner, MD, Oncologist, Lynn Cancer Institute, Boca Raton, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.