A 35-year-old male with chronic myeloid leukemia in the accelerated phase received a peripheral blood stem cell transplant from his HLA-DR-mismatched mother. Graft-versus-host disease (GVHD) prophylaxis was with short-term methotrexate and tacrolimus. After transplantation, grade II skin acute GVHD occurred and was unsuccessfully treated with bolus methylprednisolone administration. The acute GVHD progressed to grade III of the skin, gut and liver, and mycophenolate mofetil (MMF) was accordingly administered at a daily dose of 2 g. This treatment resulted in a dramatic improvement in the clinical features of the acute GVHD. The patient suffered from hemorrhagic cystitis and several episodes of cytomegalovirus antigenemia. MMF may be useful for steroid-resistant acute GVHD despite an increasing risk of viral infections.