purpose: To study the histopathologic findings, clinical course, and therapeutic outcome of patients who developed a lymphoproliferative disorder after undergoing solid organ transplantation. patients and methods: A series of 26 patients who developed a lymphoproliferative disorder after solid organ transplant during a 27-year period were studied. results: The 26 patients ranged in age from 6 to 68 years (median 42 years). The lymphoproliferative disorder was diagnosed from 1 to 211 months (median 80 months) after transplantation. The type of transplant was kidney (n = 21), heart or heart-lung (n = 4), or liver (n = 1). Most patients received azathioprine and prednisone, in addition to antilymphocyte globulin or cyclosporine, for post-transplant immuno-suppression. Eight patients had lymphoma that could be classified according to the International Working Formulation (IWF-F, IWF-G, IWF-H). Sixteen patients had polymorphic lymphoma, and 2 patients were classified as having polymorphic lymphoid hyperplasia. Patients were staged by the Ann Arbor staging system. Nine patients had stage I disease, 4 stage II, 6 stage III, and 7 stage IV. Central nervous system, lung, or marrow involvement was present in 27%, 23%, and 14% of patients, respectively. In the 17 patients studied, immunophenotype was monoclonal B-cell (n = 12), malignant T-cell (n = 2), or polyclonal B-cell (n = 3). The initial therapeutic approach was generally a reduction in immunosuppression, but, thereafter, the approach to therapy varied. In patients with localized disease, surgical excision and/or involved field radiotherapy were utilized as applicable. For patients with more extensive disease, other approaches such as high-dose acyclovir, combination chemotherapy, or alpha interferon were utilized. Overall, 15 of 26 patients (58%) responded to systemic therapy or were rendered disease-free either by surgery or radiation, including 8 (31%) with a complete remission (CR). Only 3 of 9 patients responded to chemotherapy, whereas 4 of 13 patients responded to acyclovir (including 3 patients who experienced CR). Remission duration ranged from 8 to 122 months (median 32+ months). Twenty-one of 26 patients (81%) have died. Survival ranged from less than 1 to 122 months (median 14 months). conclusion: The outcome of patients with post-solid organ transplant lymphoproliferative disorders is poor, and the optimal approach to therapy is not clear. Newer therapeutic approaches are thus needed to improve the outcome of these patients.