We report a case of acute tumor lysis syndrome appearing during chemoradiotherapeutic conditioning for allogeneic bone marrow transplantation (BMT) in a patient with chronic lymphocytic leukemia. This complication appeared in spite of prophylactic therapy and although the outcome was satisfactory, the patient required temporary hemodialysis and strict supportive care. This case emphasizes the importance of recognizing that the acute tumor lysis syndrome is not confined to high-grade malignancies.