Leukocytosis means that the white blood cell count in the peripheral blood exceeds 10×10^9/L. It is also referred to as leukocytosis. Leukocytosis can be seen in a variety of diseases, the most common being infections, which can be bacterial, viral, or fungal infections, and the patient will have symptoms of infection such as fever, cough, sputum, and sore throat. In this case, active anti-infection treatment should be given, and the white blood cell count will gradually decrease after the infection is controlled. There are also hematologic diseases such as acute leukemia, chronic leukemia, and early stage of myelofibrosis, all of which may have increased white blood cell counts. These diseases require bone marrow aspiration, flow cytometry, and bone marrow biopsy and other related tests to confirm the diagnosis. It can also be seen in malignant tumors, such as lung cancer, liver cancer, and stomach cancer, which can also cause elevated white blood cell counts.