Twenty cases of posttransplant lymphoproliferative disorders arising in solid organ allograft recipients (18 patients) or bone marrow allograft recipients (2 patients: 1 autologous; 1 allogeneic) were selected for presentation at the Society for Hematopathology Workshop. In the course of the Workshop discussions, based both on the submitted cases and the combined experience of the participants, it was possible to agree on several distinctive categories of PTLD. These include (1) early lesions, (2) polymorphic posttransplant lymphoproliferative disorders (PTLDs), (3) monomorphic PTLDs (B- and T-cell lymphomas), (4) plasmacytoma-like lesions, and (5) T-cell-rich large B-cell lymphoma/Hodgkin's disease-like lesions. Monomorphic lesions should be classified according to a recognized classification of non-Hodgkin's lymphoma, although specified in the report as PTLD. Polymorphic lesions should be carefully evaluated for clonality; by immunophenotyping; and, if necessary, analysis of antigen-receptor and Epstein-Barr virus (EBV) genomes. Minimal pathological evaluation should include routine morphology, immunophenotyping on fresh tissue (flow cytometry or frozen section), and preservation of tissue for molecular genetic analysis. Analysis of the presence of EBV can be useful in establishing whether early or equivocal lesions represent PTLD (EBV+) or unrelated processes, but is not required in most cases. The pathologist can make an important contribution to the management of patients with PTLD by providing a complete diagnostic evaluation of the biopsy specimens (this is the least expensive part of the care of a transplant patients, not a place to try to cut costs) and making sure the attending physicians understand the special issues in management of PTLD.