Basophilia (basophilic leukocytosis) is an increase in the content of basophils more than 150 in 1 μl of blood or more than 1% of the total number of leukocytes. It often accompanies eosinophilia, since basophils and eosinophils are jointly involved in many pathological reactions. The causes of basophilia are allergic, inflammatory, oncohematological diseases. There are no specific manifestations, the clinical picture is determined by the underlying pathology. Measurement of the level of basophils is carried out in the general blood test. To eliminate basophilia, it is necessary to treat the disease against which it arose.