Posttransplantation lymphoproliferative disorder (PTLD) is an uncommon but often fatal complication of solid organ transplantation that occurs in ∼3% of patients. To determine the relative importance and relationship of potential risk factors for PTLD before transplantation (i.e., Epstein-Barr virus [EBV] serostatus of the recipient and the cytomegalovirus [CMV] sero-status of the recipient and the potential donor) and the principal risk factor after transplantation (immunosuppression with antilymphocyte antibody), we analyzed the findings for the first 381 consecutive adult nonrenal transplant recipients seen at Mayo Clinic. In the absence of other risk factors, the incidence rate of PTLD for EBV-seronegative recipients was 24 times higher (95% confidence interval [CI]: 6.2, 89) than that for EBV-seropositive recipients. The additional risk factors of therapy with OKT3 for rejection and CMV seromismatch (i.e., a negative recipient and a positive donor) each further amplified this risk four- to sixfold. Together, all three risk factors acted synergistically to increase the incidence rate of fatal and/or CNS PTLD by a factor of 654 (CI: 368, 1,162) compared with the low incidence rate (.458 cases per 100 person years) when none of these risk factors were present. Pretransplantation determination of recipient EBV and CMV serostatus can identify a subgroup of patients whose risk for severe PTLD may preclude transplantation. © 1995 by The University of Chicago. All rights reserved.