A classification scheme based on our current knowledge, is presented on the next page. The number of new cases is split evenly between acute and chronic leukemia. This is the reverse of that seen in adult leukemia. There is no apparent predilection for race or sex among the myeloid leukemias. There is approximately a 20 fold increase incidence of leukemia in children with Down's syndrome .
Etiology is unknown in most cases of leukemia.
There is a loss of feed-back control, resulting in an over proliferation of immature and often functionally abnormal hematopoietic cells. It is likely that leukemic cells produce cytokines or growth factors that inhibit normal hematopoietic growth. In most instances the leukemic cells remain functionally immature. Thus while retaining the capability to proliferate, they remain, for the most part, in the marrow space, replacing normal marrow elements . The doubling time of leukemic cells can be as short as 4 days because of an espicially large growth fraction.