Leukocytosis or an increase of white blood cells in the peripheral blood is a frequent anomaly. It is important to immediately distinguish if it is a benign or malignant leukocytosis and the analysis of the blood formula is the first step. The presence of abnormal cells such as blast cells is an alarm which must immediately consider the diagnosis of leukemia, and medullogram has to be performed. The presence of lymphoma cells will lead to the conclusion of lymphoproliferative disorder and the presence of myeloid precursors to a myeloproliferative disorder. However, a benign leukemoid reaction should be excluded. If there are no abnormal cells in the differential count, it will be necessary to distinguish hyperleucocytosis from myeloid origin (neutrophilies, eosinophilies, basophilies) from lymphoid origin. In the latter case, it is the lymphocytic phenotype that will confirm the malignancy by the presence of a clone of T or B lymphocytes but also by the monomorphic appearance of lymphocytes. In cytologically pleomorphic lymphocytosis, lymphoid reactions related to viral infections, autoimmune diseases or drug-related lymphoid reactions will be sought. The different causes are discussed in detail.