Pluripotent stem cells of hematopoiesis are included among CD34+ cells in the blood and bone marrow. After granulocyte-colony stimulating factor (G-CSF) mobilization, 1-2% of the mononuclear cells in the blood are CD34+ cells, which can be obtained by leukapheresis. We performed CD34+ progenitor cell transplantation in two children with severe aplastic anemia (SAA) who lacked HLA-matched donors. The donors were treated with G-CSF, 600 micrograms/body/day subcutaneously, for 4-5 days. CD34+ cell selection was performed from the apheresis concentrate with mouse anti-CD34 antibody 9C5 and magnet beads coated with sheep anti-mouse IgG1. After the transplantation, the patients received tacrolimus to prevent graft-versus-host disease (GVHD). G-CSF was given to both patients. A mean number of 4.96 x 10(6) CD34+ cells per kilogram of body weight were transplanted. The hematopoietic recovery after the CD34+ cell transplantation was rapid, except for platelets, and acute GVHD was less than or equal to grade I. Case 1, who demonstrated mixed chimerism, anemia and thrombocytopenia after the graft, received a second transplant with intensified preconditioning, and now sustains complete and stable hematopoiesis after a follow-up of 314 days posttransplant. Although Case 2 showed early rejection and received a second transplant, sustained engraftment was never achieved. However, the patient's own hematopoiesis appeared. For SAA patients who do not have HLA-matched donors, this type of approach seems to be a feasible and useful method. However, an intensified preconditioning regimen to overcome the high likelihood of rejection should be employed.