A 34-year-old woman with a diagnosis of hemophagocytic lymphohistocytosis (HLH) received a double umbilical cord blood transplantation following a myeloablative chemotherapy preparative regimen with busulfan and cyclophosphamide. HLH is a rare, potentially fatal hematologic disorder characterized by the overactivation of histocytes and T lymphocytes, leading to organ infiltration and acute illness. On day 25 post-transplantation, the patient required a platelet transfusion for a platelet count of 6,000 per ml (normal range = 150,000-450,000 per ml). The patient's blood type prior to the cord blood transplantation was B positive and, although both umbilical cord blood donors were O positive, the patient was still B positive per blood bank testing on that day. Although the recipient of an allogenic stem cell transplantation will eventually become the blood type of the donor, the time for this process to occur varies for each person. That process must be monitored by the blood bank for the purpose of cross-matching blood products to decrease hemolysis as much as possible. The patient was premedicated with the facility's standard for platelet transfusions: acetaminophen 650 mg and diphenhydramine 25 mg about 30 minutes prior to the platelet transfusion.