Of 84 renal transplants performed in our center since 1986, six recipients (7.1%) developed posttransplant lymphoproliferative disorder (PTLD). All received quadruple immunosuppression with Minnesota anti-lymphoblastic globulin or anti-thymocyte globulin, methylprednisolone, cyclosporine, and azathioprine or mycophenolate mofetil. Five were seronegative for Epstein-Barr virus (EBV) when they received their renal transplant. All patients received prophylactic acyclovir treatment postrenal transplant and none developed a cytomegalovirus (CMV) infection. All patients were positive for EBV by serology and polymerase chain reaction at the time of diagnosis of PTLD. Clinical features at presentation included fever (6/6), adenopathy (4/6), hypertrophied adenoids (4/6), liver involvement (2/6), and allograft involvement (2/6), 2-78 months (4/6<6 months) postrenal transplant. Histopathology of PTLD tissue revealed T cell rich/ Hodgkin disease-like B cell PTLD in one patient, polymorphic PTLD in four, and monomorphic (large B cell lymphoma) PTLD in one. Immunophenotyping of the PTLD biopsy specimen revealed predominant T cells in three, mixed B and T cells in two patients, and B cell in one. No aneuploid populations were identified by flow cytometric DNA ploidy assay. DNA from the PTLD tissue revealed weak to moderate IgH gene rearrangement in four of six patients but no T cell receptor beta-chain or c-myc gene rearrangement on Southern blot analysis. The child with monomorphic (large B cell lymphoma) PTLD was clonal with lambda light chain restriction on immunophenotyping. Treatment consisted of reduced immunosuppression and ganciclovir/ acyclovir in all patients. CMV hyperimmune globulin was used as an adjunctive therapy in two patients. Chemotherapy was needed in only one patient. A single rejection episode occurred in two children following reduction in immunosuppression, which reversed following intravenous methylprednisolone therapy. PTLD resolved in all patients and at present all patients are alive with functional grafts 2-54 months post diagnosis. Our experience suggests that reduced immunosuppression and anti-viral treatment is adequate in most cases of PTLD, but chemotherapy and hyperimmune globulin therapy may be beneficial in cases resistant to first-line therapy. Since all but one of our patients were EBV seronegative at the time of transplant, vigilance is especially important for early detection of PTLD in this group of the pediatric renal transplant population.