A rapidly developing cancer of the blood-forming tissues, particularly the bone marrow, that causes overproduction of immature white blood cells known as lymphoblasts. The disease is also known as acute lymphocytic leukemia because it affects the development in the bone marrow of the normal white blood cells known as lymphocytes. The lymphoblasts multiply rapidly and replace the lymphocytes. The abnormal cells also populate the bone marrow, causing the bone marrow to fail and the number of normal blood cells to decrease. Untreated, acute lymphoblastic leukemia leads to death within months, usually from infection (caused by poor immunity because of the low number of normal white blood cells) or bleeding (caused by the abnormally low number of platelets, which have a key role in blood clotting).