Posttransplantation lymphoproliferative disorder is a serious complication of organ transplantation. This disorder has been linked to Epstein-Barr virus infections and can involve the transplanted organ. In the past the diagnosis of posttransplantation lymphoproliferative disorder in heart transplant recipients involving the transplanted organ was made primarily at autopsy. A case of a patient with posttransplantation lymphoproliferative disorder in whom the diagnosis was made initially by endomyocardial biopsy with confirmation by application of molecular techniques on mediastinal lymph node tissue and who was subsequently treated successfully is reported.