The normal value of white blood cells in infants is higher than that of normal adults, so the normal value of adults should not be used to measure whether the white blood cells of infants are elevated. The normal value of white blood cells in infants varies with age, and the younger the age, the higher the value. The average is less than 20 x 10^9/L in January, about 11 x 10^9/L in the first year of life, and decreases year by year after the age of 2, until about 10 years of age when it approaches that of an adult. Specifically, the normal value of leukocytes for children is (5-12) × 10^9/L; for newborns it is (15-20) × 10^9/L. A baby can only be judged as having leukocytosis if his or her leukocyte level is higher than this standard. At this point it is necessary to clarify the cause of the leukocytosis. Common infant leukocytes (mainly including monocytes, lymphocytes, neutrophils) increase are: (1) pediatric leukocytosis is most commonly granulocytosis, caused mainly by bacterial infections, followed by fungal and other infections. (2) Pediatric eosinophilia is mainly caused by parasitic diseases, followed by allergic diseases. (3) Pediatric lymphocytosis is mainly seen in viral infections, pertussis, tuberculosis, infectious lymphocytosis, infectious mononucleosis, and lymphocytic leukemia. Because of the numerous clinical causes of elevated leukocytes and the different clinical manifestations, no generalizations can be made. Detailed medical history, physical examination, laboratory tests, and medical imaging are required to determine the causative factors and to treat them.