PROTOCOL OUTLINE: Patients receive oral busulfan twice a day on days -9 to -6; cyclophosphamide IV over 1 hour on days -5 to -2; etoposide IV over 4 hours on days -5 to -3; and anti-thymocyte globulin IV twice a day on days -2 and -1 and days 1 and 2. Patients undergo allogeneic hematopoietic stem cell transplantation on day 0. Filgrastim (G-CSF) is administered subcutaneously beginning on day 1 and continuing until blood counts recover. Patients receive graft versus host disease prophylaxis with methotrexate IV on days 1, 3, 6, and 11 and cyclosporine IV over 1-4 hours (orally once the patients resumes eating) every 12 hours (every 8 hours for pediatric patients) starting on or prior to day -3 and continuing up to 1 year.