We report a patient with post-transplant lymphoma who was treated by renal allograft nephrectomy, discontinuation of immuno-suppressive therapy, and initiation of acyclovir administration. Despite these measures he appeared to have progressive lymphoma. Had a biopsy and cultures not been done, the diagnosis of aspergillosis would have been missed and the patient might have been treated with chemotherapy, with a potentially lethal outcome. Data from the Cincinnati Transplant Tumor Registry indicate that of 662 patients treated for posttransplant lymphoma, 277 patients died of cancer and 137 died of other causes, of which infection was a major factor. This case emphasizes the importance of proper work-up of patients with apparently progressive lymphomas.