The highest dose level proposed for this study will exceed the reported toxic level for busulfan in the alloSCT setting. Patients will be followed closely for toxicity and strict stopping rules have been included. Eligibility criteria will exclude patients with prior history of hepatotoxicity or viral hepatitis. Potential hepatotoxic agents will not be allowed just prior to and during the busulfan dosing period. In addition, patients who experience hepatotoxicty during pre-transplant mobilization therapy may be excluded from receiving dose-escalated busulfan therapy. Every attempt will be made to prevent or avoid hepatotoxicity.