The association between Epstein-Barr virus subtype, human leukocyte antigen class I alleles, and the development of posttransplantation lymphoproliferative disorder was examined in a group of 25 bone marrow transplant recipients. A highly statistically significant correlation was observed between the human leukocyte antigen B51 allele and development of posttransplantation lymphoproliferative disorder (P=.0016). This study provides, to our knowledge, the first evidence that the human leukocyte antigen B51 allele might predispose bone marrow transplant recipients to Epstein-Barr virus-associated posttransplantation lymphoproliferative disorder.