Unfortunately, less than 1/4th of patients with severe SCD will have a matched sibling donor that can serve as a BMT donor. This research protocol proposes to study the safety and usefulness of "alternate donor transplant" (using donors other than matched siblings). We will offer this treatment to children with severe sickle cell disease that do not have a matched sibling BMT donor. Alternative donors can be family members who are slightly less than completely matched, unrelated volunteer donors who are completely matched, and donated banked umbilical cord blood that is completely or nearly completely matched.