Post-transplantation lymphoproliferative disorders (PTLDs) are primarily B-cell disorders that are thought to be Epstein-Barr virus (EBV) driven and that can occur months to years after solid organ or bone marrow transplantation. A small percentage of cases have also been shown to be T-cell phenotype, but a PTLD of NK-cell type has not been previously described. We report here the case of a renal transplant recipient in whom a clinically aggressive, histologically monomorphic PTLD developed that was documented to be of an NK-cell phenotype according to paraffin section and flow cytometric immunophenotyping. Molecular-genetic analysis showed the PTLD to contain germline immunoglobulin heavy, kappa light chain, and T-cell receptor beta and gamma genes. Studies for EBV failed to demonstrate the presence of viral infection in tumor cells. Clinical follow-up showed a rapidly fatal course. To our knowledge, this is the first reported case of an EBV-negative PTLD of true NK-cell type.