Any of several types of cancer in which there is usually a disorganized sudden increase of white blood cells in the bone marrow from which all blood cells originate: With leukemia, the production of red blood cells, platelets, and normal white blood cells is decreased because they are crowded out by the leukemic cells.

As a result of leukemia, some of the other bodily organs may no longer function properly, such as the liver, spleen, lymph nodes, testes, or the brain.

Strictly speaking, leukemia should refer only to cancer of the white blood cells (the leukocytes) but in practice it can apply to malignancy of any cellular element in the blood or bone marrow, as in red cell leukemia or erythroleukemia.

Certain types of childhood leukemias respond well to treatment, which includes drugs, chemotherapy and radiotherapy.