A 17-year-old male with AML FAB M4 relapsed 4 months after myeloablative conditioning and peripheral blood stem cell transplantation (PBSCT) from an HLA-identical unrelated donor. A second PBSC harvest was infused 2 days after completion of cytoreductive therapy with mitoxantrone 7 mg/m(2)/day i.v. for 3 days (total dose 21 mg/m(2)), fludarabine 30 mg/m(2)/day i.v. for 6 days (total dose 180 mg/m(2)) and Ara-C 125 mg/m(2)/day i.v. for 5 days (total dose 625 mg/m(2)). Neutrophil recovery occurred on day +10 and was associated with GVHD grade III of the skin which was treated with cyclosporin A (CsA) and prednisone. Because of fever of unknown origin and progressive fatigue combined with hypotension on day +15 after second PBSCT, echocardiography was performed which revealed a dramatic decrease in systolic function compared to the status pre-transplant. On the same day acute heart failure with consecutive ventricular fibrillation occurred. Although resuscitation was performed immediately the patient died. The autopsy revealed massive infiltration by donor CD8-positive lymphocytes with concomitant extensive damage of the heart tissue. Acute myocarditis of viral origin was excluded by in situ hybridization and nested PCR techniques. In this patient, myocardial involvement by acute GVHD seems to have triggered a fatal arrhythmia and heart failure.