Cerebrospinal fluid nucleated cells are mainly leukocytes. Normal adult cerebrospinal fluid contains only a small number of leukocytes (0-5×10^6/L), mainly monocytes. Increased leukocytes are seen in inflammatory lesions of the cerebrospinal membrane and brain parenchyma. A marked increase in leukocytes with predominantly multiple nucleated cells is seen in acute suppurative meningitis; a mild or moderate increase in leukocytes with predominantly single nucleated cells is seen in viral encephalitis; a large number of lymphocytes or monocytes are predominantly seen in subacute or chronic infections; and a large number of eosinophils can be seen in parasitic infections of the brain. Normal adult cerebrospinal fluid has a very low level of nucleated cells, and its increased level can be seen in a variety of diseases, it is recommended to consult a doctor in time to clarify the diagnosis and timely treatment.