OBJECTIVE Lymphoproliferative disorders in solid organ recipients are usually of B-cell type and have rarely been described in childhood. This study describes the development of T-cell acute lymphoblastic leukemia (ALL) in a child occurring 6 years after renal transplantation. METHODS An 11-year-old boy had received a renal allograft from his father at 5 years of age. He was receiving imuran, prednisone, and cyclosporin A prophylaxis for graft rejection after transplant until T-cell ALL was diagnosed. Although an acute Epstein-Barr virus (EBV) infection was noted at the time of diagnosis, the EBV genome was not detected by Southern blot analysis and polymerase chain reaction (PCR) in the leukemic cells. RESULTS A large mediastinal mass and malignant pleural effusion were noted at diagnosis. Leukemic cells of his bone marrow and pleural fluid expressed T-cell antigens with unique cytogenetic features, including add(1)(p36.1), del(11)(q14), and monosomy 7. EBV serology was consistent with a recent infection but EBV genome was not detected by Southern blot and PCR analysis in his leukemic cells. Human T-lymphotropic virus-I (HTLV-I) antibody titer was negative. He has been on chemotherapy for 9 months, maintaining his first remission. CONCLUSIONS Malignancies developing after renal transplantations are usually lymphoproliferative disorders and of B-cell origin. In the majority of these patients, EBV plays an etiologic role. Although adult T-cell leukemia developing during immunosuppressive treatment in renal transplant recipients has been reported, T-cell leukemia after transplant in pediatric patients has not been reported to date. This case is unique in terms of the patient's age, the T-cell immunophenotype, the cytogenetic features, and the absence of an EBV genome within the leukemic cells despite an acute EBV infection before diagnosis.