We evaluated the engraftment of donor cells from unrelated cord blood into adult patients with severe aplastic anemia (SAA) and the outcome of allo-CBSCT (cord blood stem cell transplantation). Nine patients were conditioned with decreased dosage of immunosuppressive agents of CTX (60 mg/kg) and ALG (120 mg/kg). The prophylaxis of GVHD consisted of standard CsA and MTX. Patients have a media age of 25.3 yr (range: 15-37), and a median weight of 57.2 kg (range: 52.5-60) at the time of transplantation. Cord blood searches were all conducted at Guangzhou Cord Blood Bank. The engraftment state of the donor cells into recipients was confirmed by microsatellite DNA fingerprinting and fluorescent quantitative PCR analysis. Engrafted evidence has been found in seven patients involved by biomolecular analyses showing donor-recipient mixed chimerism post-transplant which was stable and persistent. After a median follow up of 32.2 months (range: 4-69), seven patients were alive and disease free. This study shows that durable donor-recipient stable mixed chimerism can be achieved by unrelated CBSCT in patients with SAA. Umbilical cord blood could be employed as a source of hematopoietic stem cell for adult transplantation.