Objective To investigate the effect of allogeneic hematopoietic stem cell transplantation (allo-HSCT) with intensified conditioning regimen followed by rapidly tapering immunosuppressants and sequential minimal residual disease (MRD)-guided donor lymphocyte infusion (DLI) post-transplantation on outcome of blastic plasmacytoid dendritic cell neoplasm (BPDCN).Methods Two cases of BPDCN from January 2009 to May 2011 in Nanfang hospital were diagnosed according to 2008 WHO classification of tumours of haematopoietic and lymphoid tissues.Case 1 initially presented with typical cutaneous involvement and was promptly diagnosed with CD+4CD+56LCA+TdT+CD+43 BPDCN by skin biopsy.Case 2 was recognized as acute lymphocyte leukemia and acute non-lymphocytic leukemia,which was diagnosed to BPDCN at recurrence through flow cytometry analysis.Total-body-irradiation plus cyclophosphamide based intensified conditioning regimen were followed by allo-HSCT from sibling donor.Graft-versus-host disease (GVHD) prophylaxis consisted of cyclosporine and methotrexate.Anti-thymocyteglobulin was included additionally for haploid donor allo-HSCT.Multi-color labeling flow cytometry was performed to monitor MRD.Rapidly tapering of prophylactic immunosuppressants and sequential MRD-guided donor lymphocyte infusion (DLI) were performed to control relapse of primary malignancy.Results Two cases of BPDCN received allo-HSCT from sibling donor after intensified conditioning regimen.Both patients achieved complete remission and complete donor engraftment.Case 1 survived refractory acyclovir-resistant Epstein-Barr virus viremia benefiting from preemptive treatment with rituximab and DLI-induced grade Ⅳ acute GVHD,but died of thrombotic microangiopathy mixed with diffuse alveolar hemorrhage and sepsis on +243 days.Case 2 relapsed just 2 months after allo-HSCT despite DLI and rapidly tapering of CsA,died of sepsis followed by diffuse intravascular coagulation on +101 days.Conclusion BPDCN is characterized with typical cutaneous and/or bone marrow involvement with CD4+CD+56CD+123CD+43 blastic plasmacytoid dendritic cell and highly aggressive clinical course.Allo-HSCT seems to be a promising treatment for early phase of aggressive BPDCN aided with MRD monitoring and DLI,but it deserves more intensive researches to promote outcome of advanced staged BPDCN.