Mucopolysaccharidosis type I (MPS-I) is an inborn error of metabolism with progressive multisystem involvement. Hurler syndrome is the most severe form of MPS-I that causes progressive deterioration of the central nervous system with ensuing death. This study reported the therapeutic effect of allogeneic hematopoietic stem cell transplantation (allo-HSCT) on Hurler syndrome in one case. The patient was a 25-month-old boy. He underwent allo-HSCT. The donor was his elder sister whose HLA-B locus was not matching. The reduced-intensity of BuCy conditioning regimen in allo-HSCT for this patient was as follows: busulfan 3.7 mg/kg daily at 9 to 6 days before transplantation, cyclophosphamide 42.8 mg/kg daily at 5 to 2 days before transplantation, and rabbit antithymocyte globulin 3.5 mg/kg daily at 1, 3, 5, and 7 days before transplantation. Human granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood stem cells (CD34+ cells 12.8 x10(6)/kg) were infused and cyclosporine (CSA), short-course methotrexate, daclizumab and mycophenolate mofetil (MMF) were administered to prevent graft-versus-host disease (GVHD). Complete donor-type engraftment was confirmed by Short Tandem Repeat-Polymerase Chain Reaction (STR-PCR) on day 14 after transplantation. Neutrophil and platelet engraftment occurred on days 11 and 19 after transplantation respectively. Only grade I regimen-related toxicity of live and gastrointestinal tract occurred. GVHD and graft failure were not observed. After transplantation, the clinical symptoms and the neurocognitive function were greatly improved in this patient. It was concluded that allo-HSCT was effective for the treatment of MPS-I. The reduced-intensity conditioning regimen was helpful to decrease the regimen-related toxicity. Sufficient immunosuppressive therapy and adequate hematopoietic stem cells infusion may be beneficial to the donor cell engraftment and reducing the incidence of graft failure and GVHD.