Familial hemophagocytic lymphohistiocytosis (FHL) is a disorder of immune homeostasis characterized by fever, cytopenias, hepatosplenomegaly, and coagulopathy. We studied the outcomes of 13 FHL patients who underwent the first unrelated cord blood transplantation (UCBT) after non-myeloablative conditionings. The major regimen consisted of fludarabine (FLU; n 5 12) 1 melphalan (MEL; n 5 11) ± low-dose total body irradiation (TBI 2-4 Gy; n 5 6). The median age at presentation and period to UCBT were 6 and 5 months, respectively. Central nervous system (CNS) disease developed in one infant at diagnosis, and in two others until UCBT. HLH activity was controlled in all but one at the time of UCBT. Ten patients had early engraftment on median day 21 with no grade >2 treatment-related toxicity and two controllable grade >2 acute GVHD. Two patients with early rejection successfully underwent subsequent UCBT after myeloablative conditioning. Two others had late graft failure following mixed donor chimerism. Two deaths occurred from HLH; early liver failure and late CNS disease. Of 11 FLU1MEL-conditioned patients, the frequency of disease-free complete engraftment was higher for MEL ( 120 mg/m 2 )1TBI, or high-dose MEL (180 mg/m 2 ) than for others (83% vs. 25%, p 5 0.036). The FLU1MEL-based non-myeloablative regimen was acceptable for FHL infants undergoing UCBT, although further studies will be needed for confirmation.Hemophagocytic lymphohistiocytosis (HLH) is a disorder of immune homeostasis characterized by fever, cytopenias, hepatosplenomegaly, hyperferritinemia, and disseminated intravascular coagulopathy [1]. The lifethreatening condition arises from uncontrolled activation of proliferating lymphocytes, hemophagocytosing macrophages, and hypercytokinemia. The genetic basis of primary HLH resides in the defective cytotoxicity of Tcells and natural killer cells, although secondary HLH can occur in patients with infection, malignancy, or autoimmune disease. Familial HLH (FHL) is an autosomal recessive disease. Affected patients develop HLH mostly in childhood as the sole phenotype of the disease and require allogeneic hematopoietic stem cell transplantation (SCT) for a complete cure. The causal genes of FHL account for the structure, function, and metabolism of cytotoxic granules, such as PRF1 (FHL2), UNC13D (FHL3), STX11 (FHL4), and STXBP2 (FHL5) [2]. A potential risk of developing HLH is also a part of the clinical expressions of X-linked lymphoproliferative disease (XLP1 and XLP2), Chediak-Higashi syndrome, and Griscelli syndrome [3].Recent advances in SCT procedures have improved the survival rate of patients with FHL who undergo SCT after myeloablative conditioning, which now range from 45 to 65% [4][5][6][7][8][9][10][11]. However, there remain critical problems with SCT for FHL; early death from treatment-related events often associated with venoocclusive disease, and neurological sequelae in survivors. The early age at diagnosis also raises a clinical dilemma concerning the timing, preparative regimen, and al...