Researchers examined the correlation between fludarabine use and mixed chimerism (MC) and/or secondary graft failure (SGF) with recipient- or donor-type chimerism in allogeneic transplantation for aplastic anemia. Patients with aplastic anemia aged greater than 15 years who were treated and achieved engraftment were included in the study. Analyses were performed in four groups: MC that did not require either granulocyte-colony stimulating factor (G-CSF) or transfusion support (group 1; n=26), MC (not SGF) that required G-CSF and/or transfusion support (group 2; n=16), SGF with MC or complete recipient-type chimerism (group 3; n=19), and SGF with complete donor-type chimerism (group 4; n=17). Overall one- and five-year survival in patients without MC or SGF (n=340) was 90.4% and 83.5%, respectively; rates were not significantly different in groups 1 and 2, but overall survival was inferior in groups 3 and 4. Fludarabine use and the absence of irradiation in conditioning were correlated with SGF development with MC or complete recipient-type chimerism. Fludarabine use in conditioning was correlated with SGF with complete donor-type chimerism.