Chronic graft versus host disease (GVHD) is a common late complication of hematopoietic stem cell transplantation. Polymyositis is a rare manifestation of chronic GVHD after donor lymphocyte infusion (DLI). Patients with both polymyositis and myocarditis have not been reported to date. Here, we report an 18-year-old female patient who developed polymyositis and myocarditis after a DLI. The patient developed the symptoms of fever, generalized myalgia, dysarthria, and asymptomatic sinus tachycardia at DLI day +102, and 17 days after the discontinuation of immunosuppressants. The laboratory testing showed elevated muscle enzymes, and the electromyographic examination revealed myopathic abnormalities compatible with the diagnosis of myositis. The muscle biopsy showed CD8+ T cell infiltration of the muscle fibers. The electrocardiogram (ECG) showed sinus tachycardia with an incomplete right bundle branch block, anteroseptal ST elevation and lateral ST depression. Echocardiography showed mild hypokinesia of the left interventricular septal wall without evidence of infection or leukemic relapse. The patient was immediately treated with 60 mg/day of prednisone and tacrolimus after the diagnosis of polymyositis and myocarditis, apparently associated with chronic GVHD. The cardiac and muscle enzymes decreased and the ECG normalized after immunosuppressant treatment. The follow-up ECG 2 weeks after initiation of therapy was normal.