 Candidates for this trial will include patients with high risk forms of ALL, AML or CML, non-Hodgkins lymphoma, or myelodysplastic syndrome for whom an allogeneic marrow transplant is clearly indicated, and patients with aplastic anemia refractory to ATG or cyclosporine treatment and who are transfusion dependent. All research participants will be conditioned for transplantation with intravenous busulfan busulfex 0.8- 1.0 mgKgdose Q6H x 10 doses, melphalan 70 mgm2dose x 2 doses and fludarabine 25mgm2day x 5 doses. Doses of busulfan will be adjusted according to plasma levels. All research participants will also receive ATG Thymoglobulin prior to transplant to promote engraftment. No drug prophylaxis againstGvHD will be administered post transplant. All research participants will also receive G-CSF post-transplant to foster engraftment. 