PROTOCOL OUTLINE: Patients with severe combined immunodeficiency (SCID) using a matched sibling donor receive allogeneic bone marrow or umbilical cord blood transplantation on day 0. Patients receive graft versus host disease (GVHD) prophylaxis with methotrexate IV on days 1, 3, 6, and 11 and cyclosporine IV on days -3 to 50.