Leukemia is cancer of the blood. The bone marrow produces defective white blood cells, called leukemia cells, which are unable to perform their normal functions. Eventually, the cancer may spread to the red blood cells as well. Leukemia is classified as acute (progressing quickly) or chronic (progressing slowly), and as lymphocytic or myelogenous, depending on the type of white blood cell affected, lymphoid cells or myeloid cells. Most childhood leukemia is acute, and acute lymphocytic leukemia (ALL) is the most common among children. Other types of leukemia that can affect children are acute myelogenous leukemia (AML) -- also known as acute nonlymphocytic leukemia (ANLL) -- and chronic myelogenous leukemia (CML). One other common type of leukemia, chronic lymphocytic leukemia (CLL), very rarely affects children.