Just before the bone marrow transplant, we must kill the cells in the bone marrow that make the abnormal red blood cells found in patients with severe thalassemia or sickle cell disease. We will do this by using 3 drugs: busulfan, cyclophosphamide, and campath-1H. Campath-1H is used to prevent the body from rejecting or refusing to let the donor blood cells grow in the body. MESNA is given with the cyclophosphamide to prevent kidney damage. Methotrexate and cyclosporin are also given to prevent graft-versus-host disease (GVHD); methylprednisolone will be given after the bone marrow infusion if the patient develops GVHD. A drug will also be given to prevent seizures (either dilantin or lorazepam).