Leukocytosis, increased number of leukocytes, belongs to one of the most frequent laboratory findings. Distinguishing of the secondary (reactive) from malignant cause is one of the most important steps in diagnostics of leukocytosis. Examination of other laboratory parameters of blood count helps us in diagnostics and informs us about the stage of erythropoiesis and thrombocytosis. In the case of malignant leukocytosis it is necessary to distinguish myeloid and lymphoid leukocytosis. At malignant lymphocytosis we utilize morphological signs of pathological cells and very often we base on examination by flow cytometry of superficial CD signs of given pathological population of cells and their cytogenetic changes. When finding blasts in peripheral blood, we immediately come to examination of bone marrow and in myeloid leukocytosis, which is often secondary cause on the basis of an infection, we think also of myeloproliferative disease as chronic myeloid leukemia, osteomyelofibrosis or myelodysplastic syndrome.