Malignant lymphoma is a frequent complication of organ transplantation. It has been suggested that such tumors arise as a result of uncontrolled proliferation of Epstein-Barr virus-infected B lymphocytes in an immunosuppressed host. Although a few cases of posttransplant lymphomas in bone marrow transplantation have been shown to be of donor cell origin, no recipients of solid-organ transplants are known to have developed lymphomas arising from donor cells. In this report, a case of diffuse high-grade lymphoma that apparently arose in the allograft of a renal transplant recipient is described. DNA fingerprinting demonstrated the tumor to be of donor origin; Epstein-Barr sequences were absent. A therapeutic trial consisting of withdrawal of immunosuppressive agents and administration of acyclovir was unsuccessful. These data support the notion that donor cells can undergo malignant transformation in solid-organ transplant recipients, and such tumors need not carry EBV genetic material.