Leukemia, acute lymphoblastic: A form of leukemia that has a sudden onset and is characterized by the presence in the blood and bone marrow of large numbers of unusually immature white blood cells that are precursors of lymphocytes (lymphoblasts). Lymphoblasts are rarely seen in the blood under normal circumstances. Abbreviated ALL. Treatment for ALL may include chemotherapy, radiation, biological therapy, and bone marrow transplantation. There is a high cure rate for ALL today, especially among children. Also known as acute lymphocytic leukemia.