Plasma cell myeloma type posttransplant lymphoproliferative disorder (PTLD) is a rare subtype of monomorphic B-cell/plasmacytic-type PTLD. Only 10 cases of monomorphic plasmacytic-type PTLD have been previously reported in pediatric transplant recipients (kidney, liver, small bowel-liver, and heart). We present a case of Epstein-Barr virus positive monomorphic plasma cell myeloma type PTLD that developed 10 months after cardiac transplant in an 18-month-old boy. The bone marrow showed replacement by about 20% to 40% lambda-restricted plasmacytoid lymphocytes and plasma cells (by immunohistochemistry and flow cytometry, respectively). His serum free lambda to kappa light chain ratio was >300, comparable to that seen in myeloma in nontransplant patients. The neoplastic cells were Epstein-Barr virus small RNA positive by in situ hybridization. He was treated with rituximab in combination with ganciclovir, intravenous immune globulin, and discontinuation of immunosuppressants. However, he succumbed to septic shock and multiorgan failure 1 month after diagnosis.