Solid-organ transplant recipients have an elevated risk for some malignancies because of the requirement for immunosuppression [1]. In particular, non-Hodgkin's lymphoma (NHL) is common and comprises one end of a spectrum of post-transplant lymphoproliferative disorder (PTLD) ranging from benign hyperplasia to lymphoid malignancy [2]. PTLD risk is influenced by the type of organ transplanted, the age and Epstein-Barr virus (EBV) serostatus of the transplant recipient, and the intensity of immunosuppression [3-9]. PTLD incidence is high immediately after transplantation, decreases subsequently, and then rises again 4-5 years from transplantation [10,11]. This incidence pattern suggests the presence of separate early-onset and late-onset PTLD subtypes. Early-onset PTLDs tend to be EBV-positive and, when extranodal, are more likely than late-onset PTLDs to be localized to the transplanted organ [12,13]. Late-onset PTLD is less likely to be associated with EBV and, overall, is more likely than early-onset PTLD to be extranodal [13,14]. The Scientific Registry of Transplant Recipients (SRTR) includes data on a large number of solid-organ transplant recipients in the United States and information on malignancies diagnosed post-transplantation. We used these data to conduct a retrospective cohort study among kidney transplant recipients to examine differences in risk factors between early-onset PTLD and late-onset PTLD.