A 53-year-old woman with myelodysplastic syndrome received a cord blood transplant because she had frequent episodes of febrile neutropenia. As a conditioning regimen for transplant, she received 12 Gy total body irradiation, intravenous cytosine arabinoside 3 g/m 2 every 12 hours on days -5 and -4, and cyclophosphamide 60 mg/kg/day on days -3 and -2. She received tacrolimus and short-term methotrexate treatment as prophylaxis for graftversus-host disease. Her cardiac function was normal before transplant. She developed acute heart failure with a mild pericardial effusion 11 days after transplant, but her failure improved with a diuretic, vasodilator, and inotropic agent. She complained of dyspnea, and chest auscultation revealed pericardial friction rubs on day 28. Massive pericardial effusion was detected by echocardiography and pericarditis was diagnosed. The pericardial space was drained by pericardiocentesis. The pericardial fluid was exudative, but no bacteria or fungi were cultured. On viral polymerase chain reaction examination, human herpesvirus-6 was detected at a level of 3 × 10 4 copies/mL in the pericardial effusion, but not in the peripheral blood. With conservative treatment alone, that did not include antiviral therapy, her symptoms disappeared on day 56. We conclude that human herpesvirus-6 reactivation may have been associated with her pericarditis.