Leukoerythroblastosis can be defined as the presence of both immature WBCs (metamyelocytes or earlier cells) and nucleated RBCs in the peripheral blood smear. Although the relative frequency of etiologies is different in different reports, approximately 25%-30% of patients with leukoerythroblastosis have metastatic tumor in the bone marrow, about 20% have leukemia, about 10% have myeloid metaplasia or polycythemia vera, and about 8% have hemolytic anemia. Severe infection, megaloblastic anemia, and severe acute hemorrhage account for about 5% each. There is a miscellaneous nonneoplastic group with relatively few cases of any single etiology comprising 5%-15% of the total.