Acute tumor lysis syndrome (TLS) is a condition that results from the rapid destruction of tumor cells accompanied with a massive release of cellular breakdown products. Acute renal failure resulting from TLS has been reported in cases of hematologic malignancies, spontaneous or treatment induced, the latter especially by chemotherapy. We present the case of a patient with diffuse large B-cell lymphoma who developed radiotherapy-induced TLS and subsequently acute renal failure. He presented with a large mediastinal tumor compressing the airway, thus causing dyspnea. After 6 Gy/3 fractions/3 days of palliative radiotherapy for the tumor, a decrease in urine volume was noted as well as rapid tumor shrinking. Because this patient died despite previous prophylaxic anti-uric acid treatment and hemodialysis, his case illustrates the need to anticipate the development of acute renal failure, even though there may be no remarkable serum uric acid elevation after the initiation of radiotherapy.