Marrow transplantation was carried out in a patient with acute myelogenous leukemia using a healthy identical twin as donor. The patient was prepared for grafting with combination chemotherapy, high dose cyclophosphamide, and 1,000 rad total body irradiation. Repopulation of marrow did not occur despite a second marrow infusion 21 days following the first attempt. The patient died 44 days after the initial marrow infusion. To our knowledge this is the first patient transplanted from an identical twin who did not achieve engraftment after high dose chemotherapy and supralethal irradiation. The survival time was well in excess of that required for evaluation of engraftment. The possible mechanisms for graft failure are discussed. The use of prophylactic interferon against viral infections is considered as the most likely cause of failure of engraftment.