Leukemia is cancer of the white blood cells. It is the most common type of childhood cancer. Your blood cells form in your bone marrow. White blood cells help your body fight infection. In leukemia, however, the bone marrow produces abnormal white blood cells. These cells crowd out the healthy blood cells, making it hard for blood to do its work. Leukemia can develop quickly or slowly. Acute leukemia is a fast growing type while chronic leukemia grows slowly. Children with leukemia usually have one of the acute types. Risk factors for childhood leukemia include having a brother or sister with leukemia, having certain genetic disorders and having had radiation or chemotherapy. Treatment often cures childhood leukemia. Treatment options include chemotherapy, other drug therapy and radiation. In some cases bone marrow and blood stem cell transplantation might help.