Most cases of childhood leukemia are acute lymphoblastic leukemia, a small proportion being myelogenous leukemia. Chronic myelogenous leukemia is rare in children, whereas chronic lymphocytic leukemia is not encountered. The prognosis of childhood leukemia has improved considerably so that approximately 80 to 90% of those having lymphocytic leukemia and 60 to 70% of those having myelogenous leukemia will recover fully. The challenge is to develop new treatments for disease groups having a poor prognosis and on the other hand to lighten the treatments for leukemias having a good prognosis.