Post-transplant lymphoproliferative disorders are mostly Epstein-Barr virus-related, B-cell tumors that develop as a consequence of immunosuppressive therapy in recipients of solid organ or bone marrow transplants. These disorders range from reactive, polyclonal plasmacytic hyperplasia to those that are morphologically and genotypically indistinguishable from typical non-Hodgkin's lymphomas. Plasma cell myeloma occurring after solid organ transplantation is rare. We report three plasma cell myeloma post-transplant lymphoproliferative disorder cases and one polymorphic, monoclonal post-transplant lymphoproliferative disorder case associated with a monoclonal serum protein. All three plasma cell myeloma post-transplant lymphoproliferative disorder cases had clinical, radiologic, and pathologic features of conventional plasma cell myeloma. The one polymorphic post-transplant lymphoproliferative disorder case was associated with an IgM monoclonal serum protein and was morphologically indistinguishable from a lymphoplasmacytic lymphoma. Three of the four cases, including the one polymorphic post-transplant lymphoproliferative disorder case, were positive for Epstein-Barr virus encoded small RNA by in situ hybridization. One patient died of plasma cell myeloma post-transplant lymphoproliferative disorder. The remaining three patients are alive: two are completely free of post-transplant lymphoproliferative disorder, and one has shown partial response to therapy. We compare the clinicopathologic features of these cases with those in the literature.