We report 12 cases of lymphomas which occurred among 1670 patients with kidney or combined renal and pancreatic transplantation. Group 1 comprised nine patients presenting with the diffuse form of the disease where immunoblasts or mature plasma cells massively infiltrated all organs. The first symptom was a viral syndrome, associated with a restriction of heterogeneity of immunoglobulins; oligoclonal to monoclonal peaks of immunoglobulins appeared about 50 days after transplantation. All patients received antilymphocyte globulins (ALG), and seven were treated with cyclosporin. EBV infection could be demonstrated in almost all patients; three EBV lymphoblastoid cell lines were established, their HLA phenotype being the same as the recipient of the graft. All patients finally died with renal and hepatic failure. Group 2 comprises three patients who presented solid B cell tumours of tonsils, lungs, and spleen at onset, extending to liver, kidney graft, lymph nodes, and brain. All received cyclosporin; two patients were treated with ALG, and one with OKT3. Immunoglobulins were polyclonal, oligoclonal, or decreased. Cell surface immunoglobulins were monoclonal on two tumours. EBV-DNA was positive within two tumours. Two patients presented EBV and CMV primary infection. CD4+T lymphocytes subsets were diminished at onset, and increased after cessation of immunosuppressive therapy. One patient died because of brain involvement; the two others are alive, one with perfect graft function. Therapy consisted of stopping immunosuppressive treatment, Acyclovir, and in two patients of group 2, monoclonal antibodies to pan-B and EBV receptor antigens.