A 32-year-old man received a cadavaric renal transplant in 1975 for end-stage renal disease and, thereafter, was treated with azathioprine and methylprednisolone for chronic immunosuppression. In 1985, he presented with fever and pancytopenia that persisted despite withdrawal of the immunosuppressive agents. Lymph node and liver biopsies demonstrated malignant lymphoma within the sinuses of the node and the sinusoids of the liver. A splenectomy was performed for persistent pancytopenia, and the spleen demonstrated malignant lymphoma of the diffuse mixed large and small cell type exclusively within the cords of the red pulp. The immunophenotype of the tumor cells was obtained by frozen section immunoperoxidase staining with monoclonal antibodies and flow cytometric analysis. The tumor cells were positive for the Pan T cell markers CD3 and CD2, but were negative for the subset markers CD4 and CD8. A DNA hybridization study conducted on the splenic tissue conclusively identified the clonal nature of the malignant T cells by demonstrating rearrangement of the T cell receptor beta gene. In spite of multiple chemotherapeutic regimens, the patient developed increasing peripheral blood involvement and died with disseminated lymphoma. This case appears to be unique in that it is the first report of a chronically immunosuppressed transplant recipient to develop a malignant lymphoma of the mature T cell type, and several of the pathologic features of the tumor have not been observed previously.