Transient Myeloproliferative Disorder (Transient Leukemia) is found in approximately 10% of newborn infants with Down Syndrome. It is characterized by the large numbers of megakaryoblasts in the peripheral blood, variable thrombocytopenia and, in a minority of cases, by a lethal course with hydrops fetalis or progressive hepatic fibrosis. Evidence is presented that this disease is truly leukemia, which, in most cases, recovers spontaneously. There is evidence that hematopoiesis is abnormal in Down Syndrome children who do not have leukemia. These abnormalities of red cells, platelets, and granulocytes are reviewed.