A 56-year-old patient received a cadaveric renal allograft because of primary cystic kidney disease. The donor was a 28-year-old man who died from head trauma. No other major illnesses were present at the time of transplantation. Immunosuppression was performed with cyclosporine A and steroids. After 3 months, the patient presented with fever and abdominal pain which was located in the region of the allograft. Ultrasonography demonstrated a tumor mass at the renal transplant hilus that was suspected to be an infected hematoma. Kidney biopsy from the cortex revealed only severe morphologic signs of cyclosporine A toxicity which was due to high cyclosporine A levels during the first 2 months after transplantation. The patient died from pulmonary embolism 6 months posttransplant. Histologic evaluation of the tumor specimens obtained at autopsy showed an extensive infiltration of the renal hilus and the medulla by a peripheral T cell lymphoma of the large-cell type. The T cell origin was confirmed by immunohistochemistry using the T cell-associated monoclonal antibodies UCHL-1 and MT1.