(1) Infection (severe, chronic, e.g., tuberculosis), esp. in children; (2) hemolysis (severe); (3) malignant neoplasms (esp. carcinoma of the breast, lung, kidney); (4) cytokines (e.g., G-CSF, GM-CSF). May be distinguished from chronic myeloid leukemia (CML) by measurement of the leukocyte alkaline phosphatase (LAP) level: elevated in leukemoid reactions, depressed in CML.