Leukemia: Cancer of the blood cells. Strictly speaking, leukemia should refer only to cancer of the white blood cells (leukocytes), but in practice it can apply to malignancy of any cellular element in the blood or bone marrow, as in red cell leukemia (erythroleukemia). Treatment may involve chemo'therapy, radiation therapy, biological therapy, and/or bone marrow transplantation. Also spelled leucemia. See also accelerated phase of leukemia; leukemia, blastic phase of; leukemia, chronic phase of; leukemia, hairy cell; leukemia, lymphocytic; leukemia, myeloid; leukemia, refractory; myelodysplastic syndrome.