Hairy cell leukemia (in french: leucémie à tricholeucocytes) is a distinctive form of chronic leukemia with splenomegaly, pancytopenia, presence of a particular circulating mononuclear cell, myelofibrosis and leukemic infiltration of the bone marrow. Generally, a tartrate resistant acid phosphatase activity is found in the leukemic cells. Hairy cell leukemia is the main leukemia in which the treatment is first surgical. Splenectomy has a good result for survival median and infectious risk. The disease is still mysterious for the origin of the leukemic cell; monocyte, B lymphocyte or in few cases T lymphocyte, even though a spectrum of immunological patterns pleads for the B lymphocyte origin of the leukemic cells.