The study is a Phase II, single arm, multi-center trial. It is designed to estimate the efficacy and toxicity of unrelated donor HCTin patients with SCD and high risk features who are between 3 and 16 years of age. The primary objective is to determine event-free survival (EFS) at 1 year after unrelated donor (URD) hematopoietic stem cell transplantation (HCT) using bone marrow (BM) in patients with sickle cell disease (SCD). Death, disease recurrence or graft rejection by 1 year will be considered events for this endpoint. Secondary objectives include determining the effect of HCT on clinical and laboratory manifestations or severe sickle cell disease including stroke and determining the incidence of other transplant related outcomes.