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Abstract 4520 The impact of ABO major incompatibility in hematopoietic stem cell transplantation (SCT) using conventional myeloablative conditioning has been reported to be negligible in clinical settings. To date, there are scant studies about its clinical significance using reduced-intensity nonmyeloablative SCT (RISCT). In this study we analyzed 32 cases performed HSCT using fludarabine-based RISCT (median age was 15 years old ranged from 7 months to 35 years old, 10 cases with and 22 cases without ABO major incompatibility) and 19 cases using conventional myeloablative SCT (median age was 8 years old ranged from 1 to 16 years old, 6 cases with and 13 cases without ABO major incompatibility) in our institution from January, 2005 to April, 2010. All patients using RISCT were non-malignant disorders including 13 patients with chronic granulomatous disease, 11 with aplastic anemia, and 5 with severe congenital neutropenia. On the other hand, all patients with conventional myeloablative SCT were pediatric malignant disorders including 8 patients with acute lymphoblastic leukemia, and 7 with acute myelogenic leukemia. As shown in Table, in RISCT, the number of red blood cell (RBC) transfusion after SCT was significantly higher and the last day of RBC transfusion was also significantly longer in patients with ABO major incompatibility than those in patients with ABO compatibility. Notably, two patients with ABO major incompatibility had prolonged delay of RBC engraftment and pure red cell aplasia due to the presence of high titer of anti-A antibody. Furthermore, the prolonged recovery of platelet and neutrophil were also observed in patients with ABO major incompatibility (Table). On the other hand, in HSCT using conventional myeloablative conditioning, no differences in the recovery of RBC and platelet were noted irrespective of ABO incompatibility. These data suggest that RISCT with ABO major incompatibility affects the engraftment and recovery of multi-lineage blood cells. Thus, the alternatives of donors may be an important role in transplant-related morbidity of RISCT with ABO incompatibility.TablePatients using RISCTABO major incompatibilityCompatibleIncompatibleP valuesPatients NumbersN=22N=10The number of RBC transfusion* (time)2.1 ± 3.113.6 ± 26.7 Disclosures: No relevant conflicts of interest to declare.