Leukocytosis and leukemia belong to two concepts. Patients with leukemia may have elevated white blood cells, but high white blood cells do not necessarily mean leukemia. Leukocytosis is a condition in which the white blood cell count in routine blood tests exceeds the normal reference value (usually >10×10^9/L). Common causes of leukocyte elevation are as follows: 1. bacterial infections: such as bronchitis, pneumonia, gastroenteritis, appendicitis, etc.; 2. viral infections: such as EBV infection leading to infectious mononucleosis, which can also cause leukocytosis; 3. drug factors: such as glucocorticoids, granulocyte colony-stimulating factor, which can cause leukocyte elevation; 4. autoimmune diseases: such as dry syndrome, thyroid function hyperthyroidism, etc., can manifest as elevated leukocytes at the onset, and acute and chronic leukemia, myeloproliferative disorders, true erythropoietic disorders, primary thrombocytosis, myelodysplastic syndromes and other hematologic disorders, can also manifest as leukocytosis. In conclusion, there are many causes of elevated leukocytes and leukemia cannot be diagnosed solely on the basis of increased leukocytes. However, if the leukocyte growth is very fast and the white blood cell value is very high, along with anemia, infection and bleeding, then you need to be alert to leukemia. In general, if elevated white blood cells appear after routine blood tests, it is recommended to visit a regular hospital and identify the cause as soon as possible to avoid delaying the disease.