If you agree to take part in this study, you will receive a test dose of busulfan by vein over about 60 minutes. This low-level test dose of busulfan will help the doctor decide what dose of busulfan you will receive during the study. You will most likely receive this in an outpatient clinic during the week before you are in the hospital. If it cannot be given to you as an outpatient, you will be admitted to the hospital on Day -11 (11 days before your stem cells are returned to your body) and the test dose will be given on Day -10. With stem cell transplants, the days before you receive your stem cells are called minus days (Days -11, -10, and so on). The day you receive the stem cells is called Day 0. The days after you receive your stem cells are called plus days (Days +1, +2, and so on).