A. In the first year after organ transplantation, recipients are at the greatest risk of developing lymphoproliferative diseases (PTLDs), which are induced most often by Epstein-Barr virus (EBV) infection, and patients should therefore be screened prior to or at the time of transplantation for EBV antibodies. B. In the rare cases (<5%) where the recipient is EBV seronegative, he or she has a 95% likelihood of receiving an organ from an EBV-seropositive donor, which translates into a high risk of primary EBV infection with seroconversion soon after transplantation. In such cases, the recipient should receive a prophylactic antiviral treatment with acyclovir, valacyclovir or ganciclovir, starting at the time of transplant and lasting for at least 3 months. The specific recommendations given for CMV prophylaxis could be applicable in this situation. C. The treatment of PTLD should be based on accurate pathology with extensive cell markers and phenotyping. The treatment modalities are as follows. Reduction of basal immunosuppression in all cases (either maintain only steroids, or decrease by at least 50% the anti-calcineurin drugs and stop other immunosuppressive drugs). In the case of EBV-positive B-cell lymphoma, antiviral treatment with acyclovir, valacyclovir or ganciclovir may be initiated for at least 1 month or according to the blood level of EBV replication when available. In the case of rare lymphomas from the mucosal-associated lymphoid tissue (MALT) with positive Helicobacter pylori, full eradication of H. pylori should be carried out with a validated protocol. Subsequent H. pylori prophylaxis should be implemented to avoid relapse. In the case of CD20-positive lymphomas, treatment with rituximab, a chimeric monoclonal antibody directed against CD20, should be carried out with one i.v. injection per week for 4 weeks. In the case of diffuse lymphomas or improper response to previous treatment, CHOP chemotherapy should be used alone or in combination with rituximab. The CHOP regimen is cyclophosphamide, doxorubicine, vincristine and prednisone. Complete cessation of immunosuppression with or without graft nephrectomy should also be considered.