Posttransplantation lymphoproliferative disorder (PTLD) is a well- recognized complication of organ transplantation. The aim of this study, performed over 9 years, was to examine the histopathological findings, clinical course, and outcome of patients who, having undergone orthotopic liver transplantation (OLT), developed PTLD. The sample included 7 adult liver allograft recipients (1.7%), 4 men and 3 women, with a mean age of 53 years (range, 40 to 61 years) who developed PTLD 1 to 36 months post-OLT (mean, 6 months). Four patients received either antithymocyte globulin as primary immunosuppression or OKT3 for steroid-resistant cellular rejection. Four patients had localized hepatic tumor with or without regional lymph node involvement, 2 patients had extralymphoreticular disease (head of pancreas and chest wall), and 1 patient had spleen and lymph node involvement. All tumors were B-cell lymphomas; three polymorphic and four monomorphic. Clonality was assessed by immunostaining for kappa and lambda and gene rearrangement. Monoclonality was found in 4 patients and polyclonality in 2 (1 of whom progressed to monoclonality); in 2 patients, clonality could not be determined. Immunohistochemistry findings for the presence of the Epstein- Barr virus (EBV)-determined nuclear antigen and the latent membrane protein I were noted in lymphoma tissue in 6 patients. Immunosuppressive therapy was decreased in all patients. Polyclonal tumors were treated with acyclovir (1 patient is in complete remission and 1 patient died), and monoclonal tumors with systemic chemotherapy (2 patients are in complete remission and 2 patients died). One patient was treated with monoclonal antibodies (CD20) but failed to respond, and 1 patient was treated with excision and is in complete remission. The mortality rate was 43%; for the remainder, median survival is 21 months (range, 10 to 42 months). We conclude that PTLD may represent early after OLT. EBV has a special role in the pathogenesis, combined with immunosuppressive therapy. The outcome is poor, and new therapeutic approaches are needed.