A 50-year-old woman presented with leukocytosis, anemia, and thrombocytopenia in June 2013. She was diagnosed with de novo acute myeloid leukemia with the t(16;21)(q24;q22) translocation. She received an allogeneic hematopoietic stem cell transplant from an HLA-DRB1 locus-mismatched unrelated donor in June 2014. The myeloablative preparative regimen consisted of cyclophosphamide at 60 mg/kg for 2 days and total body irradiation at 12 Gy. On Day 55, she was treated with prednisolone at 20 mg/day for acute GVHD (Grade III; Skin Stage 2, Gut Stage 2, Liver Stage 0) and gradually improved. She had fever, myalgia in the upper limbs, and asymptomatic sinus tachycardia on Day 145. Laboratory tests showed elevated CK, CKMB, aldolase, and troponin I. Electromyographic examination revealed myopathic abnormalities compatible with the diagnosis of myositis. Electrocardiography showed tachycardia and anteroseptal ST elevation, and echocardiography showed hypokinesia of the left interventricular septal wall without evidence of infection or leukemia relapse. She was immediately treated with 40 mg/day prednisolone after the diagnosis of polymyositis and cardiomyopathy, associated with chronic GVHD. The polymyositis and cardiomyopathy improved promptly after the administration of prednisolone and the patient remains in remission with a current maintenance program of prednisolone at 5 mg/day.