You will receive a test dose of busulfan by vein over 60 minutes as an outpatient usually during the week before admission. If the test dose cannot be given as an outpatient, you will be admitted to the hospital on Day -9 for IV fluids and the busulfan test dose. This low-level "test" dose of busulfan is to check how the level of busulfan in your blood levels changes over time. This information will be used to decide the next dose needed to reach the target blood level that matches your body size.