As part of a randomized phase II trial from the Bone Marrow Transplant Clinical Trials Network (BMT-CTN), MMF 1 g twice daily plus corticosteroids was given as 1 of 4 possible therapies; MMF pharmacokinetics were studied in the context of acute GVHD response. This cohort included 32 patients and MMF pharmacokinetic sampling occurred in weeks 1 and 2. Targets for therapeutic concentration of the active metabolite mycophenolic acid (MPA) were set at 0.5 mg/mL or unbound MPA at 0.015 mcg/mL. One of the most significant findings was that approximately half of the patients did not achieve these drug levels, and drug levels had a direct impact on response rates.