Leukemia, hairy cell: A rare type of chronic leukemia in which the abnormal white blood cells appear to be covered with tiny hairs when examined microscopically. The hairy cells are malignant B lymphocytes. There may be too few normal blood cells of all types because of an excess of leukemic cells in the bone marrow. The deficit of different types of normal blood cells can lead to anemia, easy bleeding, and a tendency to infection. Treatment may include chemotherapy, biological therapy, and surgery (to remove the enlarged spleen). In some cases, bone marrow transplantation is done.